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AS ChemistryUnit 5: Organic Chemistry II

Section 1: Alcohols Part 1: Introduction

Task 1

Can you solve the anagrams and complete the gaps in the notes below using your knowledge of IGCSE chemistry?

The alcohols consist of a veloltcany bonded hydroxyl group __________ attached to a carbon loseenkt. The general formula for this ooousmlhgo series is:

The most widely used alcohol is __________, CH3CH2OH.

Ethanol production

Two main processes are currently used to make ethanol. The process chosen depends on coocimen factors and on the end use of the ctdporu.

Fermentation

The process uses a sugar such as slugeco. In fermentation, the sugar is slowly mopsodcede by __________ to produce ethanol and __________:

The enzymes are made by singroams called __________.

At low pratmusteera the reaction is __________. At high temperatures, the yeast cannot revivus and the enzymes are __________. The process is normally carried out at a temperature of __________oC.

When the concentration of ethanol reaches about __________%, the yeasts and their enzymes no longer inofcnut and fermentation stops. __________ is used to achieve higher concentrations of ethanol in the end product.

Direct hydration

The second process used to product ethanol is the direct hydration of __________. __________ is currently available in huge quantities from the __________ of crude oil. __________ and __________ react together in the presence of a __________ acid catalyst at __________K and __________ MPa pressure.

The overall equation is represented in the following chemical equation:

Cambridge A-level Centre


This latter method is currently favoured for the intoproudc of ethanol for ilstrandui use. However, as this method uses ethene as a raw airmetal, it may become less ropalup compared to fermentation when oil supplies begin to run out.

Task 2

Complete the table below to compare the two methods

Method Rate of reaction

Quality of product

Raw material

Type of process

Renewability of raw material

hydration ethene

fermentation

sugars

Classifications and reactions

There are three main classes of alcohols: primary, secondary and tertiary. As for the haloalkanes, this classification is based on the number of alkyl groups (R) attached to the carbon atom bearing the –OH group.

Task 3

Draw structural formulae for each of the following substances and state whether each is a 1o, 2o or 3o alcohol:

a. butan-2-olb. 2-methylbutan-2-olc. cyclohexanol

Summarise your findings. What structural group do primary, secondary and tertiary alcohols contain?

Primary

Secondary

Tertiary

Many reactions of the –OH functional group are the same in all alcohols (we will look at this later), independent of where it is attached to the carbon skeleton. However, the three types of alcohol differ in their reactions with mild oxidising agents such as acidified potassium dichromate(VI), K2Cr2O7.

Oxidation of alcohols



Primary alcohols are oxidised first to aldehydes, such as ethanal. Aldehydes contain the functional group RCOH.

Task 4

Use structural formulae to show the oxidation of ethanol to ethanal in the space below. Show all of the bonds in the functional groups. You may use [O] to represent the oxidising agent.

An aldehyde still has one hydrogen atom attached to the carbonyl carbon, so it can be oxidised further to a carboxylic acid. You studied carboxylic acids at IGCSE. As before, write a balanced equation to show this oxidation step.

N.B. When using [O] to represent an oxidant you must still ensure the equation balances.

The 1o alcohol, e.g. ethanol is dripped into a warm solution of the oxidising agent. The aldehyde, e.g. ethanal, which forms, immediately distils off (due to its lower boiling point), preventing further oxidation to the carboxylic acid.

Can you think of a reason why aldehydes have a lower boiling point than the corresponding alcohol?

If oxidation of ethanol to ethanoic acid is required, the reagents must be heated under reflux (to prevent the aldehyde evaporating).

2o alcohols are oxidised to ketones. This family contains the RCOR grouping.

Task 5

Can you write a balanced equation for the oxidation of propan-2-ol to propanone.



These have no hydrogen atoms attached to the carbonyl carbon and so cannot be easily oxidised further.

3o alcohols are not easily oxidised.

When orange potassium dichromate (VI) acts as an oxidising agent, it is reduced to green chromium (III) ions. This colour change can be used to distinguish 1o and 2o alcohols from tertiary alcohols.

To distinguish between 1o and 2o alcohols, you must do tests on their respective oxidation products (i.e. on the aldehydes and ketones).

ReferencesA-level Chemistry pages 335-340Chemistry in Context pages 466-468, 465-476

Learning ObjectivesCandidates should be able to:

recall the chemistry of alcohols, as exemplified by ethanol, including their oxidation to carbonyl compounds and carboxylic acids.

classify hydroxy compounds into primary, secondary and tertiary alcohols.

suggest characteristic distinguishing reactions, e.g. mild oxidation. describe the formation of carboxylic acids from alcohols and

aldehydes.



Section 1: Alcohols Part 2: Physical properties and general chemistry

Task 1

Consider the table below.

Alcohol Mr Boiling point (K)

ethanol 46 352propan-1-ol 60 371butan-1-ol 74 390propane 44 231butane 58 273pentane 72 309

i) How do the boiling points of the alcohols compare with that of the alkanes of similar Mr?

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ii) Boiling point is determined by the strength of the intermolecular forces. Can you use this information to explain any difference in the boiling points of the alkanes and the alcohols.

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iii) Why do we consider compare molecules of similar Mr rather than those containing the same number of carbon atoms?

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Solubility of alcohols in water

The small alcohols are completely soluble in water. Whatever proportions you mix them in, you will get a single solution.

However, solubility falls as the length of the hydrocarbon chain in the alcohol increases. Once you get to four carbons and beyond, the fall in solubility is noticeable, and you may well end up with two layers in your test tube.

The solubility of the small alcohols in water

Consider ethanol as a typical small alcohol. In both pure water and pure ethanol the main intermolecular attractions are hydrogen bonds.

In order to mix the two, you would have to break the hydrogen bonds between the water molecules and the hydrogen bonds between the ethanol molecules. It needs energy to do both of these things.

However, when the molecules are mixed, new hydrogen bonds are made between water molecules and ethanol molecules.

The energy released when these new hydrogen bonds are made more or less compensates for that needed to break the original ones.

Imagine what happens when you have got, say, 5 carbon atoms in each alcohol molecule.



The hydrocarbon chains are forcing their way between water molecules and so breaking hydrogen bonds between those water molecules.

The -OH end of the alcohol molecules can form new hydrogen bonds with water molecules, but the hydrocarbon "tail" doesn't form hydrogen bonds

That means that quite a lot of the original hydrogen bonds being broken aren't replaced by new ones.

Some other chemical reactions of the alcohols

Combustion

The simplest way to oxidise alcohols is to burn them. Ethanol burns with a pale blue flame.

Task 1

Can you write a balanced equation for the complete combustion of ethanol?

What could be produced in a limited supply of oxygen? Why is this a cause for concern?

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The reaction between sodium and ethanol

Details of the reaction

If a small piece of sodium is dropped into some ethanol, it reacts steadily to give off bubbles of hydrogen gas and leaves a colourless solution of sodium ethoxide.



Although at first sight you might think this was something new and complicated, in fact it is analogous to the reaction between sodium and water - something you have probably known about for years.

Task 2

Can you write balanced equations for the reactions between sodium and water, and sodium and ethanol?

Can you think where this latter reaction may be useful?

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Substitution to form halogenoalkanes

Halogenoalkanes are important intermediates in the formation of many other compounds. They can be formed from alcohols using a variety of reactions.

Reactions involving hydrogen halides

The general reaction looks like this:

Reaction with hydrogen chloride

Tertiary alcohols react reasonably rapidly with concentrated hydrochloric acid, but for primary or secondary alcohols the reaction rates are too slow for the reaction to be of much importance. A better method for preparing the chloroalkanes is given later.

Replacing -OH by bromine

Rather than using hydrobromic acid, you usually treat the alcohol with a mixture of sodium or potassium bromide and concentrated sulphuric acid. This produces hydrogen bromide in situ which reacts with the alcohol. The mixture is warmed to distil off the bromoalkane.



Replacing -OH by iodine

In this case the alcohol is reacted with a mixture of sodium or potassium iodide and concentrated phosphoric(V) acid, H3PO4, and the iodoalkane is distilled off.

Task 3

Can you think why phosphoric(V) acid is used instead of concentrated sulphuric acid?

Iodoalkanes can also be made by warming an alcohol with mixture of red phosphorus and iodine:

This then reacts with the alcohol to give the corresponding halogenoalkane which can be distilled off.

Chloroalkanes can be made by reacting alcohols with phosphorus(III) chloride, PCl3, phosphorus(V) chloride, PCl5, or sulphur dichloride oxide (thionyl chloride, SOCl2).

Reaction with phosphorus(III) chloride, PCl3

Alcohols react with liquid phosphorus(III) chloride to make chloroalkanes.

Reaction with phosphorus(V) chloride, PCl5

Solid phosphorus(V) chloride reacts violently with alcohols at room temperature, producing clouds of hydrogen chloride gas.

Reaction with sulphur dichloride oxide (thionyl chloride)



Sulphur dichloride oxide (thionyl chloride) has the formula SOCl2.

The sulphur dichloride oxide reacts with alcohols at room temperature to produce a chloroalkane. Sulphur dioxide and hydrogen chloride are given off. Care would have to be taken because both of these are poisonous. However, because they are gases they are easy to separate from the halogenoalkane product.

Elimination (when a small molecule is removed from a larger molecule – converts a single bond to a double bond)

Some alcohols can be dehydrated to alkenes. This is an elimination reaction. The conditions are 170oC in the presence of excess c.H2SO4 (or c. phosphoric acid), or by passing the alcohol vapour over a hot, porous ceramic surface. N.B. the acid must be in excess to avoid formation of an ether.

Task 3

Can you write a balanced equation for the dehydration of propan-2-ol to propene?

ReferencesA-level Chemistry pages 335-340Chemistry in Context pages 468, 472-474

Learning Objectives

Candidates should be able to:recall the chemistry of alcohols, exemplified by ethanol:• combustion• substitution to give halogenoalkanes • reaction with sodium• dehydration to alkenes



Section 2: Aldehydes and Ketones

You can differentiate between an aldehyde and a ketone by testing how easily they are oxidised. Any such test must involve the use of mild oxidising agents (to prevent side reactions such as the breaking of C-C bonds). You should know two such reagents:

Tollen’s reagent – this contains the complex [Ag(NH3)2]+. It is made in the reaction between silver nitrate solution and an excess of aqueous ammonia.

With gentle warming a silver mirror is formed in the presence of an aldehyde.

Fehling’s solution – this contains a deep blue copper complex. On warming this is reduced by aldehydes to form a red precipitate of Cu2O.

N.B. these reagents are not affected by ketones! The aldehydes are oxidised to carboxylic acids.

Reduction of carbonyl compounds

Aldehydes and ketones can be reduced to 1o and 2o alcohols respectively. These reactions are the reverse of the oxidation reactions outlined on the previous page. This does not take place readily. There are two common methods employed (for CIE the focus is on method 2):

Catalytic hydrogenation

This involves reacting the carbonyl compound with hydrogen in the presence of a nickel or platinum catalyst. This is an equivalent reaction to that involved in the hydrogenation of alkenes. If a molecule contains both types of double bond then it will become completely saturated when catalytically hydrogenated.

Task 1

Can you draw and name the reactants and products of the following reduction reactions.

butanone

prop-2-enal



Reaction with sodium tetrahydridoborate (III) (NaBH4) in methanol

NaBH4 is a source of the hydride ion, H-, and a powerful reducing agent. This reaction proceeds via nucleophilic attack on the carbonyl group. The intermediate ion which is formed produces the appropriate alcohol on acidification. No reaction occurs with alkenes which are not susceptible to nucleophilic attack.

Task 2

Can you draw and name the reactants and products of the following reactions. You can represent the reducing agent by [H].

ethanal

propanone

prop-2-enal



Formation of nitriles

The elements of HCN add to the carbonyl group of aldehydes and ketones to yield compounds known as hydroxynitriles (or cyanohydrins). These contain the following functional group:

Addition appears to involve nucleophilic attack on carbonyl carbon by the strongly basic cyanide ion; subsequently (or possibly simultaneously) oxygen accepts a hydrogen ion from a HCN molecule (or a molecule of water) to form the cyanohydrin product.

Task 3

Can you draw the mechanism for the nucleophilic addition of hydrogen cyanide to propanal.

The reaction is catalysed by the presence of CN- from the addition of a small amount of NaCN. The cyanide ion is a stronger nucleophile than hydrogen cyanide.

Reactions of this type are important because they introduce another carbon atom onto the carbon chain.

Task 4

Can you think of another type of reaction which introduces the formation of a new carbon-carbon bond? Write an equation for such a reaction.



All nitriles, R-C≡N, undergo hydrolysis to form carboxylic acids. This is achieved by refluxing with aqueous acid or aqueous alkali. Alternatively, reduction of the nitrile group produces an amine.

Task 5

Can you write equations for the hydrolysis and reduction of ethanenitrile in the space below?

Reduction is carried out using sodium and ethanol.

Testing for aldehydes and ketones

Test: Add a solution of 2,4-dinitrophenylhydrazine (2,4-DNPH).Result: a deep yellow or orange precipitate

Equation: Please copy from page 354 of ‘AS level Chemistry’.

ReferencesA-level Chemistry pages 350-354Chemistry in Context pages 482-489

Learning Objectives

Candidates should be able to:

describe the reduction of aldehydes and ketones e.g. using NaBH4. describe the mechanism of the nucleophilic addition reactions of

hydrogen cyanide with aldehydes and ketones. describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH) to detect

the presence of carbonyl compounds. deduce the nature (aldehyde or ketone) of an unknown carbonyl

compound from the result of simple tests (i.e. Fehling’s or Tollens’ reagents; ease of oxidation)



describe the formation of carboxylic acids from nitriles.



Section 3: Carboxylic acids

Carboxylic acids are compounds which contain a -COOH group.

Task 1

Can you draw structural formulae for the following carboxylic acids: methanoic acid, 2-methylbutanoic acid and hexanedioic acid?

The name counts the total number of carbon atoms in the longest chain - including the one in the -COOH group. Notice that, if you have side groups attached to the chain, you always count from the carbon atom in the -COOH group as being number 1.

N.B. Carboxylic acids are made in the lab by the oxidation of primary alcohols or aldehydes, and by the hydrolysis of nitriles.

The acidity of the carboxylic acids

Task 2

Use your knowledge of acid/base equilibria to complete the notes below.

Why are carboxylic acids acidic?

Using the definition of an acid as a "_________________", the carboxylic acids are acidic because of the hydrogen in the -COOH group.

In solution in water, a hydrogen ion is transferred from the -COOH group to a _________________. For example, with ethanoic acid, you get an ethanoate ion formed together with a ________________ion, H3O+.

This reaction is _______________ and, in the case of ethanoic acid, no more than about 1% of the acid has reacted to form ions at any one time. (This is a rough-and-ready figure and varies with the concentration of the solution.)

They are therefore _______________ acids.

The dissociation of ethanoic acid in water can be represented by the equation given below?



The pH of carboxylic acid solutions

The pH depends on both the __________ of the acid and how easily it _________ hydrogen ions from the -COOH group.

Ethanoic acid is typical of the acids where the -COOH group is attached to a simple alkyl group. Typical lab solutions have pHs in the __________ range, depending on their concentrations.

Reactions of the carboxylic acids

With metals

Carboxylic acids react with the __________ reactive metals to produce a __________ and __________. The reactions are just the same as with acids like hydrochloric acid, except they tend to be rather __________.

For example, dilute ethanoic acid reacts with __________. The magnesium reacts to produce a colourless solution of magnesium __________, and hydrogen is given off. If you use magnesium ribbon, the reaction is less __________ than the same reaction with hydrochloric acid, but with magnesium __________, both are so fast that you probably wouldn't notice much difference.

The reaction of magnesium with ethanoic acid can be represented by the equation below.

With metal hydroxides

These are simple __________ reactions and are just the same as any other reaction in which __________ ions from an acid react with __________ ions. They are most quickly and easily represented by the equation:

The reaction of ethanoic acid with sodium hydroxide can be represented by the equation below.



With carbonates and hydrogencarbonates

In both of these cases, a salt is formed together with carbon dioxide and water.

The following ionic equations represent the two general reactions outlined above.

References

A-level Chemistry pages 357-359Chemistry in Context pages 498-501

Learning Objectives


describe the reactions of carboxylic acids in the formation of salts.



Section 4: Esters

Esters have a hydrocarbon group of some sort replacing the hydrogen in the -COOH group of a carboxylic acid. We shall just be looking at cases where it is replaced by an alkyl group, but it could equally well be an aryl group (one based on a benzene ring).

A common ester - ethyl ethanoate

The most commonly discussed ester is ethyl ethanoate. In this case, the hydrogen in the -COOH group has been replaced by an ethyl group. The formula for ethyl ethanoate is:

Notice that the ester is named the opposite way around from the way the formula is written. The "ethanoate" bit comes from ethanoic acid. The "ethyl" bit comes from the ethyl group on the end.

Esters

Use the information in your textbooks and your scientific knowledge to answer the questions below.

1. Which functional group do all esters contain? Draw the displayed formula in the space below.

2. Esters are made by the reaction between which two families of compounds?

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3. Which term is used to describe this type of reaction



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4. The name of an ester comes partly from the parent alcohol and partly from the parent acid. The alcohol part of the name is placed first and is separated by a space before the acid part of the name. Can you draw full structural formulae for each of the following esters?

(a)propyl ethanoate

(b) ethyl propanoate

(c) methyl butanoate

(d) butyl methanoate

5. The four compounds above have something in common. What word could be used to describe them?

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6. Using full structural formulae, write a balanced equation for the reaction between ethanol and ethanoic acid.

7. What substance is used as a catalyst in this reaction?

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8. What happens if some ethyl ethanoate is heated with water containing sulphuric acid?

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9. What term is used for this type of reaction?......................................................

10. Why is alkaline hydrolysis usually preferred over acid hydrolysis?................

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11. Using full structural formulae, write a balanced equation for the hydrolysis of ethyl ethanoate in alkaline conditions.

12. Fats and oils are esters of which alcohol?.............................................................



13. What is saponification?..............................................................................................

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13. Esters are widely used in industry. Can you list 4 uses of esters below?

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References

A-level Chemistry pages 360-363Chemistry in Context pages 505-510

Learning Objectives




describe the formation of esters from carboxylic acids using ethyl ethanoate as an example.

describe the acid and base hydrolysis of esters. state the commercial use of esters, e.g. solvents; perfumes;

flavourings.


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