Organic Reactions

1. Addition The pattern: break a C-C multiple bond (also called a π bond) and form two new single bonds. The reaction occurs only at the carbons that are a part of a multiple bond, C1 and C2

Alkene with Hydrogen an alkaneAlkene with steam an alcoholAlkene with HX a halogenoalkaneAlkene with X2 1,2 halogenoalkane

2. Elimination The pattern: Formation of a π bond (double bond). Breaking of bonds at C1 and C2

Identify alpha carbon, which is the one with the leaving group attached to it. Identify the beta carbon, which is the one next to the alpha carbon. Choose the carbon which has the least number of hydrogens attached to it and this will be where the bond will form.

Halogenoalkane with ethanolic sodium hydroxide an alkene

3. Substitution

Halogenoalkane & sodium hydroxide an alcohol [reactivity I>Br>Cl>F

Halogenoalkane & Cyanide alkanenitrile

Halogenoalkane & ammonia alkylamine

See Sn1 and Sn2 reactions below

Alcohol & hydrogen halide halogenoalkane & water

Alcohol & sulfur dichloride oxide halogenoalkane & HCl & Sulfur dioxide

Alcohol & Phosphorus(v)chloride halogenoalkane & HCl + POCl3

Alcohol & phosphorus(III)halide halogenoalkane & H3PO3

4. Hydrolysis

The breakdown of a molecule of water, usually accompanied by an acid or base to speed up the reaction.

(Nucleophile)

Alkyl halide + Mg in dry ether(ethoxyethane) alkyl halideMg + water alkane + Mg(OH)halide

5. Oxidation

The addition of oxygen atoms to a molecule and/or the removal of hydrogen atoms from a molecule. The [O] refers to an oxidation reaction, such as by using Acidified Potassium Dichromate. Alkene & cold acidified potassium manganate VII alkane -1,2 –diol (a.k.a. glycol)Alkene & hot acidified potassium manganate VII Carbon Dioxide(carbon bonded to 2 hydrogens), Carboxylic acid(carbon bonded to one hydrogen and one alkyl group) or a Ketone(carbon bonded to 2 alkyl groups)Alcohol & potassium dichromate(VI) acidified with dilute sulfuric acid Primary alcohol aldehyde [orange to green] & waterSecondary alcohol ketone [orange to green] & waterTertiary alcohol no reaction

6. Reduction

The removal of oxygen atoms from a molecule and/or the addition of hydrogen atoms to a

molecule. The [H] refers to a reduction reaction, such as by using Sodium Tetrahydridoborate, [NaBH4] , Lithium Aluminum Hydride[ LiAlH4] or H2/Platinum

Aldehyde & [H] primary alcoholKetone & [H] secondary alcohol

Tests:A) 2,4 DNPH (dinitrophenylhydrazine) will form a deep orange precipitate if either are present.

This is a condensation reaction, where the 2 molecules combine and release a molecule of water.

B) Tollens’ Reagent (aqueous silver nitrate in excess ammonia). An aldehyde will display a mirror image in the test tube as the silver is reduced. The ketone test tube will remain colorless.

C) Fehling’s solution (Copper(II) Sulfate a blue solution). An aldehyde will display a brick red color. The ketone test tube will remain colorless.

Mechanisms of Organic Reactions

1. Homolytic Fission – Both atoms leave with an electron that formed the covalent bond. The species produced are called free radicals. They go through an initiation (free radical formation), propagation (chain reaction) and termination (combination) steps. Alkanes with halogens in sunlight.

2. Heterolytic Fission – The more electronegative atom takes both the electrons in the covalent bond. All of the following reactions encompass this type of fission.

Lewis Acids and Bases

A Lewis acid is an electron-pair acceptor, which is also known as an electrophile (electron-loving) in Organic Chemistry. The electrophile has a vacant orbital in its valence shell and is electron poor. Examples are: H2 , Br2 , I2 , H2O, HBr, NH4

+ , BH3 HCN, CH2 I2, H3O+, OsO4 and anything with a positive charge.

A Lewis base is an electron-pair donor, which is also known as a nucleophile (electron-loving) in Organic Chemistry. The nucleophile has an unshared pair of electrons or pi bond (seen in double and triple bonds) available for bonding and are electron rich. Examples are: RS, I, CN - , Br- , OH- , NH, CH3CO2 , CH3O, HS , NH3 , H2O. A nucleophile with a negative charge is more reactive than one without and is a stronger base.

The only way to truly understand this concept is of course to draw out the Lewis structures, showing the un-bonded pairs of electrons.

[Nucleophile] [Electrophile]

In the diagram above notice Ammonia has available electrons for bonding and the Boron Trifluoride has a vacant orbital in its valence shell. The curved arrow shows the donation of the pair of electrons from Nitrogen to Boron to form a covalent bond.

Nucleophilic substitution reactions

The nucleophile [Nu] which is electron rich will react with the electron poor compound and substitute itself forming a new bond. This causes a bond to be broken as the species leaves, known as the leaving group [LG]. Notice the leaving group takes the electrons with it. This is displayed in the reaction of halogens in sunlight.

Nucleophilic addition reactions

The attack is by the CN which is a nucleophile. Notice the positive carbon and negative oxygen ,as it is more electronegative than the carbon, in the carbonyl group. The cyanide ion which is negative attacks the positive carbon and the negative oxygen then attacks the positive hydrogen. Also note there is regenerated cyanide ions present in the solution. This can then be further reduced by LiAlH4 to yield a primary amine or hydrolyzed ay an acid to yield a carboxylic acid.

Halogenoalkanes, aka Alkyl halides

There are two different reaction mechanisms possible for alkyl halides, Sn1 and Sn2.

Sn1 Mechanism

Refers to a substitution, nucleophic, unimolecular (1 molecule) reaction. If the nucleophile doesn’t have a charge it will likely happen by this reaction. The rate of this reaction depends only on the concentration of one reactant. For the reaction to occur

1) the carbocation must be stable and stability increases with substitution on the carbon. tertiary > secondary >> primary

2) The solvent must be polar protic such as water, alcohols, and carboxylic acids. 

Tertiary carbon is a carbon bonded to three carbon atoms.

Secondary carbon is a carbon bonded to two carbon atoms.

Primary carbon is a carbon bonded to one carbon atom.

It has a two step process:

1) Slow loss of the leaving group, which generates a carbocation intermediate then2) Rapid attack of a nucleophile on the electrophilic carbocation to form a new bond.

Note the attack can occur from either side so half of the product will have original conformation and the other half will be inverted.

Sn2

Refers to a substitution, nucleophilic, bimolecular (2 molecules) reaction. The rate is dependent on both the concentration of the substrate and the nucleophile. Charged nucleophiles proceed through this reaction. This reaction is favored by polar aprotic solvents (polar enough to dissolve the substrate and nucleophile but do not hydrogen bond with the nucleophile such as: Acetone, DMSO, Acetonitrile and DMF. This reaction is completed in one step as a rapid attack of a nucleophile on the electrophile(bond forms) and removal of the leaving group (bond breaks), the product is inverted. The rate of reaction is as follows: Primary > Secondary >> Tertiary

Extra information

Molecular formula – shows actual numbers of each type of atom in a molecule

Displayed formula – shows actual bonds present in a molecule

Skeletal formula – only shows lines up and down where each point indicates a carbon

Single bonds – Alkanes, are known to have a sigma bond present, 120 degrees

Double bonds – Alkenes, are known to have a sigma bond and pi bond present, 109.5 degrees

Triple bond – Alkynes, are known to have a sigma bond and two pi bonds present.

Structural isomers – compounds with the same molecular formula but different structural formula

Positional isomers – compound whose functional group is in a different location.

Functional isomers – compound whose functional group is altogether different.

Chain isomers – compound whose has the same molecular formula but the carbon skeleton is different.

Sterioisomers – cis – same atoms bonded on the same side

Trans – same atoms bonded on the opposite side

Optical isomers – a molecule that exhibits chiralty does not have a plane of symmetry and is not superimposable on its mirror image. It is also known as an enantiomers.

chiral center – a molecule that contains a carbon atom bonded to four different atoms, which are mirror images of each other. The maximum number of stereoisomers that are possible for a compound with a chiral center is 2n

Alkanes

Structural Formula – Cn H2n +2

They are non-polar or weakly polar molecules, which are attracted to each other by weak Van der Waal’s forces. They will not be attacked by nucleophiles or electrophiles. They are soluble in all non-polar solvents (like dissolves like) such as benzene, ether, chloroform and carbon tetrachloride. The boiling point, melting point, density and viscosity increase as the length of the carbon chain increases. Branched chains will have lower boiling points than straight chains. Oil is made up of alkanes, cycloalkanes and aromatic compounds. Fractional distillation involves breaking down the alkane by boiling points. The heavier alkanes are further broken down by “cracking”. This will yield a lower molecular weight alkane(for fuel) and and an alkene (for starting products such as plastics).

Alkenes

Structural Formula – Cn H2n

They are also known as olefins, having a lower density than water. C1 to C4 are gases, C5 to C15 are liquids and >C16 are solids. The boiling point, melting point and viscosity increase with an increase in the length of the carbon chin. Branched chains will have lower boiling points than straight chains. They are colorless and have a higher reactivity than alkanes.

Alkyl halides

Structural Formula – Cn H2n+1 X

They are also known as halogenoalkanes. The boiling points increase with increasing atomic weight of the halide and are much higher than an alkane with the same carbon skeleton. They are insoluble in water but are soluble in organic solvents. They are colorless when pure and have a pleasant odor. Go through Sn1 and Sn2 reactions (see above). Used to produce PVC and hydrofluorocarbons.

Alcohols

Structural formula – R-OH

The boiling point and solubility in water increase with the number of hydroxyl groups (OH). They tend to have a high boiling point. They are used to make esters. Methanol is and anti-freeze. Ethanol is the alcohol in alcoholic beverages.

Primary – OH group is attached to the primary carbon (straight chain)

Secondary – OH group is attached to a secondary carbon (1 branch). Naming is either prefixed by 2- or sec-

Tertiary – OH group is attached to a tertiary carbon(branched on every side of the carbon). Naming is either prefixed by 3- or tert-

Carboxylic acids

Structural formula R-COOH

The central carbon of the functional group is doubly bonded to one oxygen and singly bonded to another and is sp2 hybridized.

They are polar molecules and can form hydrogen bonds with other relavent molecules. Acids containing more than 6 carbons are not soluble in water due to the hydrogen bonding. They have a high boiling point which increases with the amount of carbons present. They are soluble in less polar molecules such as ether, alcohol and benzene. Acidity increases with the increasing negativity of the electron withdrawing group.

Acidity – RCOOH > HOH > ROH > HC ≡ CH > NH3 > RH