Alkanes are rarely prepared from other types of compounds because of economic reasons. However, ignoring financial considerations, alkanes can be prepared from the following compounds:

1. Unsaturated compounds via catalytic reduction

2. Alkyl halides via coupling (Wurtz reaction)

3. Alkyl halides via Grignard reagent

4. Alkyl halides via reduction

Although organic chemists refer to the above diagrams as “equations,” they are not balanced. In addition, not every product formed is shown. These diagrams are reallyreaction schemes.

General Method of Preparation of Alkanes (Paraffins):Alkanes can be prepared by the following methods:1. From unsaturated Hydrocarbons2. From Haloalkanes

By Wurtz Reaction By reduction

3. By the Reduction of Aldehydes and Ketones4. From Grignard’s Reagent5. From salts of Carboxylic acids

By Kolbe’s electrolytic method By Heating Na-salt of Carboxylic acid

6. From Metal Carbides 1.       From unsaturated HydrocarbonsAlkanes can be prepared by the catalytic hydrogenation of unsaturated hydrocarbons in the presence of catalyst ‘Ni’ or ‘pt’ at 200⁰C to 300⁰C.

2.       From Haloalkanes

By Wurtz Reaction:

When alkyl halides are heated with sodium metal in the presence of dry ether, alkanes are obtained (generally having double number of C-atoms than in alkyl halides). This reaction is known as Wurtz reaction and used for the preparation of symmetrical alkanes.

By Reduction of alkyl halides (RX)

Haloalkanes (R-X) when heated with reducing agents like; LiAlH4/ether, Pd/H2, Pt/H2, Zn/conc. HCl, alkanes are produced.

R-X R-H + HX3.       By the Reduction of Aldehydes and Ketones          Aldehydes and ketones can be reduced into alkanes in the presence of reducing agents: amalgated zinc and conc. HCl.

4.     From Grignard’s Reagent Hydrolysis of Grignard’s reagent in the presence of ether gives alkanes. dry etherRMgX + H2O —————> R-H + Mg(OH)Xalkyl magnesium halide (alkane) (Hydroxy magnesium halide) 5.     From salts of Carboxylic acid

By Kolbe’s electrolytic method

Electrolysis of aqueous conc. solution of sodium or potassium salt of carboxylic acid gives alkanes. >>RCOONa—-Sodium salt >>RCOOK—–Potassium salt RCOONa——–> RCOO- + Na+

anion cation

During electrolysis; Electrode reaction occurs asAt anode: RCOO- - e ——–> RCOO ———> R-R + 2CO2

unstable alkaneAt cathode: Na+ + OH- ———> NaOH

By Heating Na-salt of Carboxylic acid

When Na-salt of carboxylic acid is heated with soda lime (NaOH & CaO), alkane is obtained having one carbon less than salt by removal of a molecule of CO2. This reaction is also known as decarboxylation.

RCOONa + NaOH R-H + Na2CO3

Eg:

Sodium ethanoate (methane)This is the principle reaction for laboratory preparation of methane gas.  6.        From Metal Carbides           Metal carbide like aluminium carbide (Al4C3) and beryllium carbide (Be2C) reacts with pure water to produce methane. Al4C3 + 12H2O ———-> 4Al(OH)3 + 3CH4

Related Notes:

Read more: http://notes.tyrocity.com/preparation-of-alkanes/#ixzz3dKqaLmok 

Follow us: tyrocity    on Facebook   CrackingThis page describes what cracking is, and the differences between catalytic cracking and thermal cracking used in the petrochemical industry.

IntroductionCracking is the name given to breaking up large hydrocarbon molecules into smaller and more useful bits. This is achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst. The source of the large hydrocarbon molecules is often the naphtha fraction or the gas oil fraction from the fractional distillation of crude oil (petroleum). These fractions are obtained from the distillation process as liquids, but are re-vaporized before cracking.There is not any single unique reaction happening in the cracker. The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller hydrocarbons, some of which have carbon-carbon double bonds. One possible reaction involving the hydrocarbon C15H32 might be:

Or, showing more clearly what happens to the various atoms and bonds:

 

 

This is only one way in which this particular molecule might break up. The ethene and propene are important materials for making plastics or producing other organic chemicals. The octane is one of the molecules found in petrol (gasoline).

Catalytic crackingModern cracking uses zeolites as the catalyst. These are complex aluminosilicates, and are large lattices of aluminium, silicon and oxygen atoms carrying a negative charge. They are, of course, associated with positive ions such as sodium ions. You may have come across a zeolite if you know about ion exchange resins used in water softeners. The alkane is brought into contact with the catalyst at a temperature of about 500°C and moderately low pressures.The zeolites used in catalytic cracking are chosen to give high percentages of hydrocarbons with between 5 and 10 carbon atoms - particularly useful for petrol (gasoline). It also produces high proportions of branched alkanes and aromatic hydrocarbons like benzene. The zeolite catalyst has sites which can remove a hydrogen from an alkane together with the two electrons which bound it to the carbon. That leaves the carbon atom with a positive charge. Ions like this are called carbonium ions (or carbocations). Reorganization of these leads to the various products of the reaction.

 

 

Thermal crackingIn thermal cracking, high temperatures (typically in the range of 450°C to 750°C) and pressures (up to about 70 atmospheres) are used to break the large hydrocarbons into smaller ones. Thermal cracking gives mixtures of products containing high proportions of hydrocarbons with double bonds - alkenes. Thermal cracking does not go via ionic intermediates like catalytic cracking. Instead, carbon-carbon bonds are broken so that each carbon atom ends up with a single electron. In other words,  free radicals  are formed.

 

 

Reactions of the free radicals lead to the various products.

ContributorsGeneral methods of preparation of

alkanes A. From Alkenes. Hydrogenation (“reduction”) of alkenes.

The addition of hydrogen to alkenes is known as a “hydrogenation” or “reduction”. In this reaction a molecule of hydrogen is added to the alkene molecule at the site of unsaturation i.e. where the double bond is. This is achieved under mild conditions when a catalyst is used to bring about this change.

Suitable catalysts for this reaction are : (a)  Raney Nickel,     (b) Pd on C    or (c) Pt on CThe conditions to be used can be moderate temperatures and pressures although catalytic hydrogenations often occur at room temperature and pressure. (Any increase in pressure is bound to bring about the expected reaction in better yields – Le Chatelier). 

This is by far the most important way of generating alkanes and is in many ways the easiest reaction to carry out.

 

 

B. From Carboxylic Acids and their salts - by decarboxylation reactions i.e. removal of CO2

(i) Using   Sodalime .

 When a carboxylic acid, or its salt (sodium, potassium or calcium salts are commonly used) is heated strongly with sodalime (which is essentially sodium hydroxide mixed with calcium oxide to give a non-deliquescent solid), the carboxylic acid looses CO2 and gives the alkane :

or by way of a generalized representation of this reaction :

where R represents any alkyl (or even aryl) group, and NaOH here specifically refers to sodalime. (Some authors prefer to use Ca(OH)2 as the reagent in sodalime then giving CaCO3 as the inorganic product)

This reaction gives the required alkane albeit in low yields. The problem being that it is physically difficult to get an infinitely intimate mixture of solids for reaction to proceed efficiently when the heating commences. 

(ii) By   Electrolysis . (The Kolbe Process)

The electrolysis of aqueous solutions  of carboxylic acid salts similarly gives a decarboxylation of the anion  (at the anode) and two alkyl residues (i.e. groups) combine together to give an alkane.

or in general :

The limitation with this technique is that it only produces alkanes that have an even number of carbons. If alkanes with an odd number of carbon atoms are needed, mixtures of carboxylate salts will have to be used. Such procedures would however also make it possible for other, perhaps undesirable, products to be formed. The nature of the alkanes is very much dependent on the chance collisions of the various alkyl radicals present close to the anode where the decarboxylation processes take place.

C. From Haloalkanes.

(i) By reduction of haloalkanes - using an “active” (atomic?) form of hydrogen.

Haloalkanes can easily be made to give the alkanes if these are exposed to an environment where hydrogen atoms are generated. Such conditions are found for example when sodium/mercury or aluminium/mercury amalgams are in contact with ethanol. Similarly, a zinc/copper couple with ethanol gives the same result, as would nickel/aluminium + NaOH or zinc/mercury + HCl.

General reaction :

Sources of H : Na/Hg + C2H5OH ; Al/Hg + C2H5OH; Cu/Zn + C2H5OH;

Ni/Al + NaOH; Zn/Hg + HCl.

(ii) Halogen removal via Grignard reaction.

In this reaction, the haloalkane is first made to give the Grignard reagent as an intermediate and this is then hydrolysed (with water!) to give the alkane. This is not the best use of Grignard chemistry but it does give the alkane as a product if one so wishes.

or

(iii) Dehalogenation  using metallic sodium . The Wurtz Reaction

In this procedure, where the haloalkane is heated with metallic sodium in an inert solvent such as ether, the haloalkane loses its halogen and two radicals then come together to give an alkane with a longer carbon chain. (In fact, the chain doubles in length). 

for which the generalized reaction would be :

In this reaction, same as for one of the decarboxylation reactions, only even numbered alkanes can be made. If the alkanes with an odd number of carbons have to be produced, mixed haloalkanes must be used. However, this will again then give rise to a number of products where the different alkyl groups come together to give the final alkane. Hence, a mixture of RX and R’X yields RR, RR’ and R’R’ as the alkanes.

D. From Alcohols.

Aggressive reduction of alcohols by very strong reducing agents, will remove the alcohol functionality (i.e. the OH group) replacing it with an H atom. A Reducing agent that is good enough for this transformation is red P + conc. HI(aq) heated under pressure.

Reaction represented in a general way would be :

E. From Carbonyl compounds   i.e. aldehydes and ketones – By strong reducing agents.

Using the same conditions for reduction of alcohols also gives alkanes from the carbonyl compounds such as aldehydes (RCHO) and ketones (RCOR). 

i.e.

Special synthetic methods for methane .

a. From Aluminium carbide.

The reaction of aluminium carbide with water gives methane and aluminium hydroxide as the only products.

  b. Fischer –Tropsch  Process ( Ind)

Industrial synthesis of methane can be achieved using CO and hydrogen (both easily obtained from water and carbon) using Nickel as catalyst and the reaction can be carried out at a modest 300oC.

General methods of preparation of alkanes(i) Decarboxylation :

(a) Decarboxylation means removal of CO2 from molecules having - COOH gp.

(b) Saturated monocarboxylic acid salt of sodium potassium on dry distillation with soda lime gives alkane.

RCOONa   R-H

or RCOONa +

NaOH   R-H + Na2CO3

(c) The alkane formed always contains one carbon atom less than the original acid.

(d) The yield is good in case of lower members but poor for higher members.

(e) Soda lime is prepared by soaking quick lime CaO in caustic soda solution and then drying the products. It is generally written as NaOH + CaO. Its reaction is milder than caustic soda. Otherwise the reaction will occur violently. Also CaO used alongwith NaOH keeps it dry (NaOH is hygroscopic) to aid fusion.

(f) The decarboxylation of sodium formate yields H2.

HCOONa +

NaOH   H2 + Na2CO3

CH3COONa +

NaOH   CH4 + Na2CO3

 methane

(ii) By hydrogenation of alkenes : Sabatier and senderen's method :

(a) Alkenes and alkynes on catatlytic hydrogenation give alkanes

CH2 = CH2 + H2   CH3-CH3

CH≡CH + 2H2   CH3-CH3

(b) Catalyst Ni is used in finely divided form. If Pt or Pd are used as catalyst, reaction occurs at normal temperature. Also some times Raney nickel is used as catalyst. It is obtained by boiling Ni-AI alloy with NaOH, when AI dissolves leaving Ni in finely divided state. The filtered, washed and died Ni is known as Raney Nickel. Raney Ni is effective at room temperature and atmospheric pressure.

(iii) Reduction of alkyl halides(haloalkanes):

(a) Alkyl halides on reduction with nascent hydrogen form alkanes.

R-X   R-H + HX

(b) The nascent hydrogen may be obtained by any one of the following

(i) Zn + HCI

(ii) Zn + CH3COOH

(iii) Zn-Cu couple in ethanol

(iv) Red P + HI

(v) Al-Hg + ethanol

(c) Alkyl halides can also be reduced catalytically to alkane by H2/Pd or LiAIH4or by H2/Ni.

(d) The yields are generally high and the hydrocarbons formed are pure.

Note : Zn-Cu couple is prepared by adding Zn granules in aqueous CuSO4solution where copper is deposited on the Zn pieces.

(iv) Kolbe's electrolysis method:

(i) Alkanes are formed,

on electrolysis of concentrated aqueous solution of sodium or potassium salt of saturated mono carboxylic acids

(ii) Electrolysis of an acid salt gives symmetrical alkane. However, in case of mixture of carboxylic acid salts, all probable alkanes are formed.

R1COOK +

R2COOK   R1-R2 + 2CO2 + H2 + 2NaOH

(R1-R1 and R2-R2 are also formed).

(v) By Grignard reagents:

(i) Organic compounds in which a metal atom is directly linked to carbon atom are known as organometallic compound.

e.g. HC≡CNa, (C2H5)4 Pb, (C2H5)2 Zn

(ii) Alkyl or aryl magnesium halide (R-MgX) are also called Grignard reagents or organometallic

compounds.

(iii) Grignard reagent on double decomposition with water or with other compounds having active H(the hydrogen attached on O, N, F or triple bonded carbon atom are known as active hydrogen) give alkane.

(vi) Wurtz reaction:

(a) A solution of alkyl halide in ether on heating with sodium gives alkane.

R-X + 2Na + X-

R   R-R + 2NaX

(b) An alkyl halide on Wurtz reaction leads to the formation of symmetrical alkane having an even number of carbon atoms.

(c) Two different alkyl halides, on Wurtz reaction give all possible alkanes.

CH3X + Na + C2H5X → CH3CH2CH3 + CH3CH3 + CH3CH2CH2CH3

The different steps are:

CH3X + 2Na + C2H5X → CH3CH2CH3 + 2NaX

CH3X + 2Na + C2H5X →CH3CH3 + 2NaX

C2H5X + 2Na + C2H5X → C2H5C2H5 + 2NaX

(d) The separation of mixture into individual members is not easy because their boiling points are near to each other and thus Wurtz reaction is not suitable for the synthesis of alkanes containing odd number of carbon atoms.

(e) If Zn is used in place of Na, the reaction is named as Frankland method.

(f) Limitations of wurtz reaction :

(a) Methane can not be obtained by this method

(b) The reaction fails in case of tertiary halides

(g) Mechanism : The mechanism of Wurtz reaction is although not clear however two mechanisms are proposed for this reaction.

(a) Involving intermediate formation of an organometallic compound:

RX + 2Na → [RNa] + NaX

Intermediate

RX + [RNa] → R-R + NaX

(b) Involving intermediate formation of free radicals:

RX + Na → [R] + Nax

Free radicals

R + R → R-R

Alkane

(vii) By Reduction of Carbonyl compounds :

(i) The reduction of carbonyl compounds by amalgamated zinc and conc. HCI also yields alkanes. This is Clemmensen reduction.

CH3CHO + 2H2   CH3CH3 + H2O

CH3COOH +

2H2   CH3CH2CH3 + H2O

(ii) Carbonyl compounds may also be reduced to alkanes by Wolf Kishner reaction

(viii) By reduction of alcohols, aldehydes, ketones or fatty acids and their derivatives:

(i) The reduction of either of the above in presence of red P & HI gives corresponding alkane.

(ix) By the hydrolysis of AI or Be carbides:

(i) Only CH4 can be obtained by the hydrolysis of Be or Al carbides.

AI4C3 + 12H2O   4AI(OH)3 + 3CH4

Be2C + 4H2O   2Be(OH)2 + CH4

Note :

1. Calcium carbide reacts with water to

give acetylene.

2. Magnesium carbide, Mg2C2 reacts with water to give propyne.

3. CH4 can be obtained by passing a mixture of H2S and CS2 through red and Cu tube

CS2 + 2H2S   CH4 + 4Cu2S

(x) By hydroboration of alkenes :

Alkenes on hydroboration give trialkyl borane as a result of addition of diborane on olefinic bond. This trialkyl borane on treatment with acetic acid or propanoic acid yields alkane.

2R - CH=CH2   

2(RCH2CH2)3B   2RCH2CH3

(xi) ByCorey-House synthesis :

Alkyl chloride say chloroethane reacts with lithium in presence of ether to give lithium alkyl then reacts with CuI to give lithium dialkyl cuprate. This

lithium dialkyl cuprate now again reacts with alkyl chloride to given alkane.

CH3CH2CI + 2Li   CH3CH2Li + LiCl

2CH3CH2Li + CuI → Li(CH3CH2)2 Cu + LiL

Li(CH3CH2)2Cu + CH3CH2CI → CH3CH2CH2CH3 + CH3CH2Cu + LiCl

Acids and Bases

Many students start organic chemistry thinking they know all about acids and bases, but then quickly discover that they can't really use the principles involved. For example, many students are typically not comfortable when they are asked to identify the most acidic protons or the most basic site in a molecule. Yet this is critical since an acid will typically react at the most basic site first and a base will remove the most acidic proton first.

Remember that acidity and basicity are the based on the same chemical reaction, just looking at it from opposite sides, so they are opposites.

When evaluating acidity / basicity, look at the atom bearing the proton / electron pair first. Then you may also need to consider resonance, inductive (remote electronegativity effects), the orbitals involved and the charge on that atom.

Acidity Here are some general guidelines of principles to look for the help you address the issue of acidity:

First, consider the general equation of a simple acid reaction:

The more stable the conjugate base, A-, is then the more the equilibrium favours the product side..... 

The more the equilibrium favours products, the more H+ there is.... The more H+ there is then the stronger H-A is as an acid.... 

So looking for factors that stabilise the conjugate base, A-, gives us a "tool" for assessing acidity.

Key factors that affect the stability of the conjugate base, A-,

HF > H2O > NH3 > CH4 

Electronegativity but only when comparing atoms within the same row of the periodic table, the more electronegative the anionic atom in the conjugate base, the better it is at accepting the negative charge.

HI > HBr > HCl > HFSize.  When comparing atoms within the same group of the periodic table, the larger the atom the weaker the H-X bond and the easier it is to accommodate negative charge (lower charge density)

RCO2H > ROHResonance.  In the carboxylate ion, RCO2

- the negative charge is delocalised across 2 electronegative atoms which makes it more stable than being localised on a specific atom as in the alkoxide, RO-.

Basicity A convinient way to look at basicity is based on electron pair availability.... the more available the electrons, the more readily they can be donated to form a new bond to the proton and, and therefore the stronger base.

Key factors that affect electron pair availability in a base, B 

CH3-  > NH2

- > HO- > F-

Electronegativity but only when comparing atoms within the same row of the periodic table, the more electronegative the atom donating the electrons is, the less willing it is to share those electrons with a proton, so the weaker the base.

F- > Cl-  > Br-  > I- 

Size.  When comparing atoms within the same group of the periodic table, the larger the atom the weaker the H-X bond and the lower the electron density making it a weaker base.

 RO-  >  RCO2-  Resonance.  In the carboxylate ion, RCO2

- the negative

charge is delocalised across 2 electronegative atoms which makes it the electrons less available than when they localised on a specific atom as in the alkoxide, RO-

efinitions There are three theories used to describe acids and bases :  

Acids Bases

Arrenhius Ionise to give H+ in H2O Ionise to give HO- in H2O

Bronsted-Lowry A proton donor A proton acceptor

Lewis An electron pair acceptor An electron pair donor

Now, some terminology:

Look at this equation and see how it fits the Bronsted-Lowry and Lewis definitions.

Acidity Here are some general guidelines of principles to look for that can help you address the issue of acidity: First, consider the simplified general equation of a simple acid reaction:

 

The more stable the conjugate base, A-, is then the more the equilibrium favours the product side (Ka > 1), i.e. more dissociation of H-A

More dissociation of H-A then the stronger H-A is as an acid, or The more the equilibrium favours products, the more H+ there is.... The more H+ there is then the stronger H-A is as an acid....

So looking for factors that stabilise the conjugate base, A-, gives us a "tool" for assessing acidity.

The larger Ka implies more dissociation of HA and so the stronger the acid.

The larger Ka is, the more negative the pKa so the lower the pKa, the stronger the acid.

Key factors that affect the stability of the conjugate base, A-,  

HF > H2O > NH3 > CH4 

Electronegativity.  When comparing atoms within the same row of the periodic table, the more electronegative the anionic atom in the conjugate base, the better it is at accepting the negative charge.

HI > HBr > HCl > HF

Size.  When comparing atoms within the same group of the periodic table, the easier it is for the conjugate base to accommodate negative charge (lower charge density). The size of the group also weakens the bond H-X (note this trend should be applied with care since it only works within a group).

RCO2H > ROHResonance.  In the carboxylate ion, RCO2

- the negative charge is delocalised across 2 electronegative oxygen atoms which makes it more stable than being localised on a specific atom as in the alkoxide, RO-.

General acidity trend of common organic acids (this is a very useful sequence to remember and to be able to rationalise):

Basicity A convenient way to look at basicity is based on electron pair availability.... the more available the electrons, the more readily they can be donated to form a new bond to the proton and, and therefore the stronger base.

Key factors that affect electron pair availability in a base, B

CH3-  > NH2

- > HO- > F- Electronegativity.  When comparing atoms within the

same row of the periodic table, the more electronegative the atom donating the electrons is, the less willing it is to share those electrons with a proton, so the weaker the base.

F- > Cl-  > Br-  > I- Size. When comparing atoms within the same group of the periodic table, the larger the atom the weaker the H-X bond and the lower the electron density making it a weaker base.

 RO-  >  RCO2- 

Resonance.  In the carboxylate ion, RCO2- the negative

charge is delocalised across 2 electronegative atoms which makes it the electrons less available than when they localised on a specific atom as in the alkoxide, RO-.

General acidity trend of some common organic bases:

Note that organic chemists tend to think about bases by looking at the pKa's of their conjugate acids, i.e. think about B- by looking at the acidity of BH. The implications are that the higher the pKa of the related conjugate acid, BH, the stronger the baseb B-.

Study Tip: Note that acidity and basicity are just the reverse of each other. AND 

Therefore, both are affected by the same factors, just in opposite ways.

Walkthrough of Acid Base Reactions (2): Basicityby JAMES

in ORGANIC CHEMISTRY 1 ,  ORGANIC REACTIONS ,  UNDERSTANDING ELECTRON FLOW

Last time I started writing about acid-base reactions. We looked at this list of stabilities of anions going across the topmost row of the periodic table.

 Fluoride ion is the most stable in this series because it’s the most electronegative; carbon is the least stable because it’s the least electronegative.Because of this, we were able to say that H-F was the most acidic, because it had the most stable conjugate base.

And H-CH3 (methane)was the least acidic, because it had the least stable conjugate base.

Let’s look at the flip side of this reaction. Instead of starting with HF, H2O, H3N, and CH4 and asking how likely they are to donate a proton to a common base (water in our example) , imagine we start with the anions [ F-, HO-, H2N- and H3C- ] and have them take a proton away from a common acid (such as water).

Which reactions would be most favorable? Which would be least favorable?

The same principle applies. The less stable the anion, the more likely the reaction will be to proceed to completion. So in this case, the reaction of F- with H2O would be the least favored, because F- is the most stable. And the reaction of H3C- with H2O would be the most favored, becuse H3C- is the least stable.[A clarification: these are equilibrium reactions. So what I mean by favored here is the extent to which the equilibrium would favor the products on the right]

Notice the role that each of these anions plays in these reactions: it is accepting a proton from water, so in other words it is acting as a base.

Therefore, our whole discussion of the  “stability” of anions,  for lack of a better term, goes by another name you’re familiar with: basicity. In other words:

the more stable a lone pair of electrons is, the less basic it will be.

the less stable a lone pair of electrons is, the more basic it will be.

 

Let’s tie these two posts together with a common thread:

 For any group of acids, H-X (where X can literally be anything), the strongest acid will have the most stable conjugate base. Since stability is inversely correlated with basicity, another way of putting it is:

The stronger the acid, the weaker the conjugate base.

Today’s post is about how the opposite is also true: The weaker the acid, the stronger the conjugate base.

Next time, we’ll apply this framework to other stability trends we’ve discussed previously.

 

P.S. One last note: a common misconception students have is that “weak acids are strong bases”. Not true! Methane (CH4) is a weak acid, but it can’t act as a base – it doesn’t have a lone pair.The proper way to say it is that “weak acids have strong conjugate bases”. So the conjugate base of CH4, CH3(-) is an extremely strong base.

Next Post: Walkthrough of Acid-Base Reactions (3) – Acidity Trends

Related Posts:Making Grignard reagents

What are Grignard reagents?

A Grignard reagent has a formula RMgX where X is a halogen, and R is an alkyl or aryl (based on a benzene ring) group. For the purposes of this page, we shall take R to be an alkyl group.

A typical Grignard reagent might be CH3CH2MgBr.

The preparation of a Grignard reagent

Grignard reagents are made by adding the halogenoalkane to small bits of magnesium in a flask containing ethoxyethane (commonly called diethyl ether or just "ether"). The flask is fitted with a reflux condenser, and the mixture is warmed over a water bath for 20 - 30 minutes.

Everything must be perfectly dry because Grignard reagents react with water (see below).

Warning!  Ethoxyethane (ether) is very dangerous to work with. It is an anaesthetic, and is extremely flammable. Under no circumstances should you try to carry out this reaction without properly qualified guidance.

(I was challenged by a reader because I had previously used the word "inflammable" rather than "flammable" in this paragraph. In fact, confusingly, the two words mean exactly the same thing. However, by switching to "flammable", I have removed any possible confusion.)

Any reactions using the Grignard reagent are carried out with the mixture produced from this reaction. You can't separate it out in any way.

Reactions of Grignard reagents

Grignard reagents and water

Grignard reagents react with water to produce alkanes. This is the reason that everything has to be very dry during the preparation above.

For example:

The inorganic product, Mg(OH)Br, is referred to as a "basic bromide". You can think of it as a sort of half-way stage between magnesium bromide and magnesium hydroxide.

Grignard reagents and carbon dioxide

Grignard reagents react with carbon dioxide in two stages. In the

first, you get an addition of the Grignard reagent to the carbon dioxide.

Dry carbon dioxide is bubbled through a solution of the Grignard reagent in ethoxyethane, made as described above.

For example:

The product is then hydrolysed (reacted with water) in the presence of a dilute acid. Typically, you would add dilute sulphuric acid or dilute hydrochloric acid to the solution formed by the reaction with the CO2.

A carboxylic acid is produced with one more carbon than the original Grignard reagent.

The usually quoted equation is (without the red bits):

Almost all sources quote the formation of a basic halide such as Mg(OH)Br as the other product of the reaction. That's actually misleading because these compounds react with dilute acids. What you end up with would be a mixture of ordinary hydrated

magnesium ions, halide ions and sulphate or chloride ions - depending on which dilute acid you added.

Note:  What you need to learn about this depends on what your examiners want. The only way to find that out is to look at old exam papers and mark schemes. If you are a UK A level student and haven't got copies of these, find out how to get hold of them by going to the syllabuses page to find your Exam Board's web address.

Grignard reagents and carbonyl compounds

What are carbonyl compounds?

Carbonyl compounds contain the C=O double bond. The simplest ones have the form:

R and R' can be the same or different, and can be an alkyl group or hydrogen.

Note:  Other carbonyl compounds also react with Grignard reagents, but these are all that are required for UK A level purposes.

If one (or both) of the R groups are hydrogens, the compounds are called aldehydes. For example:

If both of the R groups are alkyl groups, the compounds are called ketones. Examples include:

The general reaction between Grignard reagents and carbonyl compounds

The reactions between the various sorts of carbonyl compounds and Grignard reagents can look quite complicated, but in fact they all react in the same way - all that changes are the groups attached to the carbon-oxygen double bond.

It is much easier to understand what is going on by looking closely at the general case (using "R" groups rather than specific groups) - and then slotting in the various real groups as and when you need to.

The reactions are essentially identical to the reaction with carbon dioxide - all that differs is the nature of the organic product.

In the first stage, the Grignard reagent adds across the carbon-oxygen double bond:

Dilute acid is then added to this to hydrolyse it. (I am using the normally accepted equation ignoring the fact that the Mg(OH)Br will react further with the acid.)

An alcohol is formed. One of the key uses of Grignard reagents is the ability to make complicated alcohols easily.

What sort of alcohol you get depends on the carbonyl compound you started with - in other words, what R and R' are.

The reaction between Grignard reagents and methanal

In methanal, both R groups are hydrogen. Methanal is the simplest possible aldehyde.

Assuming that you are starting with CH3CH2MgBr and using the general equation above, the alcohol you get always has the form:

Since both R groups are hydrogen atoms, the final product will be:

A primary alcohol is formed. A primary alcohol has only one alkyl group attached to the carbon atom with the -OH group on it.

You could obviously get a different primary alcohol if you started from a different Grignard reagent.

The reaction between Grignard reagents and other aldehydes

The next biggest aldehyde is ethanal. One of the R groups is hydrogen and the other CH3.

Again, think about how that relates to the general case. The alcohol formed is:

So this time the final product has one CH3 group and one hydrogen attached:

A secondary alcohol has two alkyl groups (the same or different) attached to the carbon with the -OH group on it.

You could change the nature of the final secondary alcohol by either:

changing the nature of the Grignard reagent - which would change the CH3CH2 group into some other alkyl group;

changing the nature of the aldehyde - which would change the CH3 group into some other alkyl group.

The reaction between Grignard reagents and ketones

Ketones have two alkyl groups attached to the carbon-oxygen double bond. The simplest one is propanone.

This time when you replace the R groups in the general formula for the alcohol produced you get a tertiary alcohol.

A tertiary alcohol has three alkyl groups attached to the carbon with the -OH attached. The alkyl groups can be any combination of same or different.

You could ring the changes on the product by

changing the nature of the Grignard reagent - which would change the CH3CH2 group into some other alkyl group;

changing the nature of the ketone - which would change the CH3 groups into whatever other alkyl groups you choose to have in the original ketone.

Why do Grignard reagents react with carbonyl compounds?

The mechanisms for these reactions aren't required by any UK A level syllabuses, but you might need to know a little about the nature of Grignard reagents.

The bond between the carbon atom and the magnesium is polar. Carbon is more electronegative than magnesium, and so the bonding pair of electrons is pulled towards the carbon.

That leaves the carbon atom with a slight negative charge.

Note:  If you aren't sure about electronegativity, you can read about it in an organic context by following this link.

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The carbon-oxygen double bond is also highly polar with a significant amount of positive charge on the carbon atom. The nature of this bond is described in detail elsewhere on this site.

Note:  If you are interested, you could follow this link to thebonding in a carbonyl group. Reading the last couple of paragraphs on that page would be enough in the present context.

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The Grignard reagent can therefore serve as a nucleophilebecause of the attraction between the slight negativeness of the carbon atom in the Grignard reagent and the positiveness of the carbon in the carbonyl compound.

A nucleophile is a species that attacks positive (or slightly positive) centres in other molecules or ions.

Note:  I have included this because one of the UK A level syllabuses says that candidates should "recall that Grignard reagents act as nucleophiles".

That is all you need to know! The mechanism is more complex than this suggests at first sight, and isn't required. You won't find these mechanisms anywhere on this site.

eagent Friday: Grignard Reagentsby JAMES

in ALCOHOLS , ALDEHYDES , ALKYL HALIDES , KETONES , ORGANIC

CHEMISTRY 2 , ORGANIC REAGENTSIn a blatant plug for the Reagent Guide and the Reagents App for iPhone, each Friday  I profile a different reagent that is commonly encountered in Org 1/ Org 2. NOTE – I said that the Reagents app wouldn’t be free forever. Today’s the last day to pick up the Reagents app for free from the App Store! Get it before the price goes up. 

Today’s reagent is one that most students have experience in making at some point or another.Grignard reagents are formed by the reaction of magnesium metal with alkyl or alkenyl halides. They’re extremely good nucleophiles, reacting with electrophiles such as carbonyl compounds (aldehydes, ketones, esters, carbon dioxide, etc) and epoxides. They’re also very strong bases and will react with acidic hydrogens (such as alcohols, water, and carboxylic acids).Similar to or the same as: very similar to organolithium reagents.Examples:Grignard reagents are made through the addition of magnesium metal to alkyl or alkenyl halides. The halide can be Cl, Br, or I (not F). It’s slightly easier to make Grignards from the iodides and bromides, however. Note what’s happening here – the magnesium is “inserting” itself between the carbon and the halide. This halide the “X” referred to when we refer to Grignard reagents as “RMgX”.

One of the most common uses of Grignard reagents is in their reaction with aldehydes and ketones to form alcohols. In the first step, the Grignard forms the carbon-carbon bond. This results in an alkoxide (the conjugate base of an alcohol). To form the alcohol, it’s necessary to add acid at the end of the reaction (in what’s called the “workup” step). This is shown here as “H3O+” (the “X” is just the counter-ion, a spectator here)

The reaction behaves similarly with ketones. Again, there’s nothing special about the Cl here – it all depends on how you made the Grignard in the first place.

Grignard reagents will also add to esters. What makes these reactions a little more complicated is that they add twice. The net result (after addition of acid) is a tertiary alcohol. This is also the case for acid halides (acyl halides) and anhydrides. One notable exception is carboxylic acids (more on that below).

Another important reaction of Grignard reagents is that they will add to epoxides to form carbon-carbon bonds. One thing to keep in mind here is that the tendency is for them to add to theless substituted end of the epoxide – that is, the less sterically hindered end. You can think of this reaction as being essentially similar to an SN2 reaction. After addition of acid, an alcohol is obtained.

Grignard reagents also add to carbon dioxide (CO2) to form carboxylates, in a reaction similar to their reactions with ketones and aldehydes. The carboxylates are converted to carboxylic acids after addition of acid (such as our trusty H3O(+) ).

Finally, since Grignard reagents are essentially the conjugate bases of alkanes, they’re also extremely strong bases. This means that sometimes acid-base reactions can compete with their nucleophilic addition reactions. One common situation where this crops up is when Grignard reagents are added to carboxylic acids. It’s easy to forget that carboxylic acids… are acids. This means that instead of adding to the carbonyl, they react with the proton instead and form the carboxylate salt.

This can also be used to convert alkyl halides to alkanes. First you treat it with magnesium, and then you treat the Grignard with a strong acid. This gives you the alkane. You can also use this to introduce deuterium (D) into molecules! The first step is to make the Grignard reagent. The second is to treat that Grignard with a deuterated acid such as D2O. This gives you the deuterated alkane!

So how does it work? The key to the Grignard reagent is actually very simple. When you think about the relative electronegativities of carbon (2.5) and magnesium (1.1), the bond between carbon and magnesium is polarized toward carbon. That means that carbon is more electron rich than magnesium and is actually nucleophilic! Here’s a closer look.

In the reaction of Grignards with aldehydes, the carbon attacks the carbonyl carbon and performs a 1,2-addition to give an alkoxide. In the second step, acid is added to give you the alcohol.

There are so many other elements to the Grignard but a limited amount of space. So I’ll leave it there. If you want more details you’ll have to check out the Reagent Guide!

P.S. You can read about the chemistry of Grignard reagents and more than 80 other reagents in undergraduate organic chemistry in the “Organic Chemistry Reagent Guide”, available here as a downloadable PDF. The Reagents App is also available for iPhone, click on the icon below!

Clemmensen Reduction

The Clemmensen Reduction allows the deoxygenation of aldehydes or ketones, to produce the corresponding hydrocarbon.

The substrate must be stable to strong acid. The Clemmensen Reduction is complementary to the Wolff-Kishner Reduction, which is run under strongly basic conditions. Acid-labile molecules should be reduced by the Wolff-Kishner protocol.

Mechanism of the Clemmensen Reduction

The reduction takes place at the surface of the zinc catalyst. In this reaction, alcohols are not postulated as intermediates, because subjection of the corresponding alcohols to these same reaction conditions does not lead to alkanes. The following proposal employs the intermediacy of zinc carbenoids to rationalize the mechanism of the Clemmensen Reduction:

 Clemmensen reduction is a chemical reaction described as a reduction of ketones (or aldehydes)

to alkanes using zinc amalgam and hydrochloric acid.[1][2][3] This reaction is named after Erik Christian Clemmensen, a Danish chemist.[4]

The Clemmensen reduction is particularly effective at reducing aryl-alkyl ketones,[5][6] such as those formed in a Friedel-Crafts acylation. With aliphatic or cyclic ketones, zincmetal reduction is much more effective.[7]

The substrate must be unreactive to the strongly acidic conditions of the Clemmensen reduction. Acid sensitive substrates should be reacted in the Wolff-Kishner reduction, which utilizes strongly basic conditions; a further, milder method is the Mozingo reduction. The oxygen atom is lost in the form of one molecule of water.