Lesmahagow High School AHChemistry Organic Chemistry& Instrumental Analysis

Lesmahagow High School CfE Advanced Higher Chemistry

Unit 2

Organic Chemistry and Instrumental Analysis

Alcohols, Ethers and Aromatics

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Alcohols

Alcohols are compounds which contain the hydroxyl group (OH ) and composed of the

elements C, H and O.

From Higher chemistry you should be able to

Name both straight and branched chain alcohols

Explain the properties of alcohols and how they arise

Understand the process of oxidation and how aldehydes and ketones can be

produced from primary and secondary alcohols.

Properties

Alcohols and ethers are isomeric and you would therefore think there could be

problems in distinguishing them. The two families however exhibit very different

chemical and physical properties.

Alcohols exhibit hydrogen bonding and so will have very high melting points and

boiling points when compared to many other organic compounds of comparable shape

and relative formula mass.

Ethers are unable do not

hydrogen bond and are only

slightly polar therefore are

compounds which can be

distinguished by their nature

of being very volatile,

flammable liquids

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Preparation of Alcohols

Alcohols can be prepared from the following reactions

Acid catalysed hydration of alkenes (shown in reactions of alkenes pg )

Substitution reactions of alkanes ( shown in haloalkanes page )

Alcohols have not only played a large part in national five chemistry but many

reactions of alcohols have previously been shown in higher chemistry.

Reactions of alcohols include

Alcohols being reacted with reactive metals to form alkoxides

Dehydration of alcohols to form alkenes

Condensation of an alcohol with carboxylic acid/acid chloride to form ester

Oxidation reaction of alcohols to form aldehydes/ ketones

Reduction reaction of aldehyde/ ketone to produce alcohols.

Reaction with reactive metals to produce alkoxides.

Reactive metals such as sodium and potassium (group 1) can be reacted with alcohols

to form alkoxides.

Na + C2H5OH —> Na+ C2H5O- + H2

Sodium ethoxide:

Dehydration of Alcohols to form Alkenes

Dehydration of an alcohol to form an alkene can be brought about by two methods

Alcohol vapour passed over heated aluminium oxide (pg )

Alcohol heated with an excess of sulfuric acid.

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Ester Formation

Condensation reactions have previously been covered in your chemistry career in

particular thinking about esterification.

Esterification is a great example of a condensation in which an alcohol is reacted

with a carboxylic acid also producing 1 mole of water

The above reaction would be carried out under the influence of a concentrated

sulfuric acid catalyst.

The general condensation reaction for the production of an ester can however be

carried out instead with an acid chloride and an alcohol. This makes the overall

esterification process faster and more vigorous.

Remember during a condensation reaction a small molecule is lost. This small

molecule does not always have to be a water molecule. In the reaction above

hydrogen chloride is produced. As the product

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Ethers

Alcohols are substances which are isomeric with ethers. Ethers are also organic

compounds with the general formula R—O—R’ where R and R’ are both alkyl groups.

Due to their low molecular masses and their lack of hydrogen bonding ethers have

low boiling point liquids which are generally insoluble in water.

Comparison with alcohols can be made not only with the melting point and boiling

point but also with the miscibility in water. Alcohols are soluble with water and the

solubility is an increasing factor with the increasing chain length of the alcohol, how-

ever ethers are insoluble. The lack of hydrogen bonding between ether and water

molecules is the reasoning behind this property and again gives us another

Distinguishable factor between the two groups of molecules.

The shape of ether molecules is very similar to that of water molecules. This allows

for the polarisation of the C-O bond.

Naming Ethers

Ethers are named as alkoxy derivatives of alkanes with the smaller alkoxy group be-

ing named as the substituent.

Instead of naming an alkyl chain with the –yl ending we change it to –oxy.

The longest alkyl group is used to name the parent alkane (2nd part of the name)

The remainder of the molecule is therefore the alkoxy group.

Ethoxypentane.

+ +

-

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Naming Ethers

Uses of Ethers

Ethers are relatively non-polar molecules as the have two alkyl groups arranged

symmetrically round the oxygen atom. Ethers will, therefore, not dissolve ionic

species as the are unable to solvate the ions effectively. However, ethers will be

able to dissolve most non-polar solvents, including most organic compounds.

The use of ethers as solvents depends on several factors:-

1) they possess little reactivity with potential solutes.

2) they dissolve many organic, but few inorganic (ionic) substances.

3) the small molecules are volatile allowing easy removal after use.

4) used for solvent extraction. Ethers, however, have two major drawbacks as solvents:-

1) they form explosive mixtures with air . 2) they form explosive peroxides on long term storage.

Formula Systematic Name

CH3 - O - CH3 methoxymethane

C2H5 - O - CH3 methoxyethane

C3H7 - O - C2H5 ethoxypropane

C2H5 - O - C6H13 ethoxyhexane

C3H7 - O - C4H9 propoxybutane

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Preparation of Ethers

Ethers can be prepared by the nucelophillic attack of an alkoxide ion on a

halogenoalkane.

When sodium (a reactive metal) a sodium alkoxide ion is formed.

The sodium methoxide is then dissolved in alcohol to yield the methoxide ion. It is

this negatively charged methoxide ion which attacks the carbon atom with a slight

positive charge in the monohaloalkane.

The product of the reaction is an ether along with the negatively charged halide ion

which has been substituted.

The preparation of ether done under reflux with athe alkoxide is an example of

nucelophillic substitution.

Na + CH3OH CH3O– Na

+ + 1/2 H2

sodium methanol sodium methoxide hydrogen

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Aldehydes and Ketones

Aldehydes and ketones are organic compounds which contain the carbonyl C=O

functional group.

These compounds we have met in higher chemistry. They are formed from the

oxidation of an alcohol. The product of oxidation (either aldehyde or ketone) is

Dependant on the type of alcohol oxidised.

Pentan—3 –one propanal

Primary alcohols are oxidised to produce aldehydes whilst secondary alcohols are

oxidised to produce ketones.

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Reactions of aldehydes and ketones

Aldehydes and ketones can therefore be reduced to produce the corresponding

alcohol from which it was initially derived.

The production of the aldehyde/ketone is performed using an oxidising agent

Acidified dichromate—orange to green colour change

Tollens reagent—silver mirror formed

Benedicts—blue to orange/red colour change

The reverse reaction which is termed reduction uses lithium aluminium hydride dis-

solved in ether as a reducing agent.

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Carboxylic Acids

Carboxylic acids contain the carboxyl functional group

These are compounds which can be either solid or liquid at room temperature which

reflects the high degree of hydrogen bonding present in the molecule.

As the carbon chain gets longer the hydrogen bonding does diminish and so the

longer the carboxylic acid the lower the melting point. As the chain length of the

carboxylic otherwise known as alkanoic acid increase the solubility also decreases

due the diminishing hydrogen bonding.

Preparation of carboxylic acids.

Carboxylic acids can be prepared in two ways

1. Oxidation of primary alcohols/aldehydes

2. Hydrolysing nitriles / esters / amides

Reactions of carboxylic acids

These organic compounds can also undergo various reactions

Formation of salts by reactions with bases or metals

Condensation reactions with alcohols to produce esters in the presence of an

acid catalyst

Reactions with amino groups to form an amide link

Reduction with lithium aluminium hydride to form primary alcohols.

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Preparation of carboxylic acids

Oxidation of primary alcohol or aldehyde

Hydrolysis of nitriles

Refluxing a nitrile with a strong acid or base brings about the hydrolysis of the

nitrile and forms a carboxylic acid.

Hydrolysis of Esters

The hydrolysis of an ester carried out in an alkaline environment is another way to

produce a carboxylic acid.

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Preparation of carboxylic acids

Hydrolysis of amides

An amide link is formed when a carboxylic acid is reacted with an amine yielding not

only the amide but also 1 mole of water. The production of the amide is clearly a con-

densation reaction and so the reverse of this must be hydrolysis.

Hydrolysis of an amide would therefore produce the carboxylic acid along with the

amine used to produce it.

The preparation of carboxylic acids is again another very important mechanism in

organic chemistry which leads to many other reactions and therefore products.

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Reactions of carboxylic acids

Formation of salts from reactions with metals

Carboxylic acids remember are examples of weak acids—ones which will not fully dis-

sociate– and so can undergo all of the normal reactions of a dilute acid.

One of these could be the production of salts. A neutralisation reaction occurs be-

tween an acid and metal or base.

2CH3COOH + Mg Mg2+(CH3COO-)2 + H2

Above shows the reaction between magnesium and ethanoic acid to produce the salt

magnesium ethanoatae.

Carboxylic acid + Metal Carbonate

2HCOOH + Na2CO3 Na+HCOO- + CO2 + H2O

Carboxylic acid + Alkali

C3H7COOH + KOH K+C3H7COO- + H2O

Condensation reactions

In organic chemistry the main reaction undergone by a carboxylic acid will be in the

condensation in which an alcohol and acid join together producing an ester. An ester

can be thought of as a derivative of an acid.

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Carboxylic acid reactions

Reaction of carboxylic acid with amino group

Another previously met reaction of acids is the reaction between the carboxyl group

and an amine. This reaction involves the formation of an amide link which has previ-

ously been met at both higher and national five level. Again another condensation re-

action in which the carboxylic acid reacts with the amino group to form an amide

along with a water molecule. An amide can also be considered as a derivative of an

acid.

An amide link is used in the manufacture of polymers as well as in polyamides more

commonly known as proteins. In the case of proteins the amide link would more likely

be known as the peptide link.

Two less familiar condensation reactions lead to less familiar products—

an acid chloride and an anhydride. These products would normally been seen more as

alternative reactants

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Amines

Amines are compounds related to ammonia (NH3) in which one or more of the

hydrogen atoms in the ammonia molecules are replaced by alkyl groups.

Secondary amine general formula = (R)2NH

Tertiary amine general formula = (R)3N

Amines can be classified as primary, secondary or tertiary. Primary amine general formula = RNH2 e.g.

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Properties of Amines

Primary and secondary amines generally have higher melting points and boiling points

than that of tertiary amines of similar molecular mass due to the hydrogen bonding

between the polar NH groups and water molecules. Primary and secondary amines will

also have higher melting points and boiling points than alkanes with similar molecular

masses for the same reason.

Short chain primary and secondary amines are also water soluble due to again the

hydrogen bonding between the polar NH group and the OH groups in water mole-

cules. As seen previous the solubility decreases as the chain length increases.

Remember no hydrogen bonding occurs in tertiary amines.

Alkane Formula

Mass

Boiling

Point / °C Amine

Formula

Mass

Boiling Point /

°C

ethane 30 -89 Methylamine

(1°) 31 -7.5

propane 44 -42 Dimethylamine

(2°) 45 7.4

butane 58 -1 Trimethylamine

(3°) 59 3.5

butane 58 -1 Propylamine (1°) 59 48.6

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Amines

Amines can be termed as bases due to the lone pair on the nitrogen atom of the NH2

group being able to accept a proton from a water molecule.

CH3CH2NH2 + H2O —> CH3CH2NH3+ + OH-

ethylamine water ethylammonium hydroxide

Note the ionisation of the base is incomplete therefore it can be said to be a

weak base.

Reactions of Amines

Amines will react (just like ammonia) with acids to form salts:

NH3(g) + HCl(g) NH4+Cl-(s)

ammonium chloride

C2H5NH2(g) + HCl(g) C2H5NH3+Cl-(s)

ethylamine ethylammonium chloride

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Aromatics

Aromatics are an enormous gourp of chemicals based on the benzene ring. This

benzene ring is super important in advanced higher chemistry and gives any molecule

it is attached to very distinctive properties.

Bonding in benzene can be described in terms of sp2 hybridisation, sigma and pi

bonds, and electron delocalisation.

Benzene’s stability is due to the presence of delocalised electrons.

The lack of double bonds in the structure explains the resistance of benzene to

undergo addition reactions.

Note: The shape of a benzene molecule is: PLANAR

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Reactions of Benzene

Most functional groups are introduced into the benzene ring by a substitution reac-

tion. During the substitution reaction the pi cloud of electrons will be disturbed

however in the final product it would be re-established.

Electrophilic substitution

Only four electrophillic substitution reactions need to be learnt for advanced higher

chemistry however there can be many other products made in this way.

The region of high electron density in the delocalised electron cloud attracts

Electophiles to the benzene ring.

Remember— electrophiles are negative loving

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Reactions of Benzene

Reactions of benzene rings are in general substitution

reactions.

Chlorination of benzene

Nitration of benzene

Sulfonation of benzene

Alkylation of benzene

Chlorination and Bromination:

Chlorobenzene or indeed bromobenzene can be produced by reacting chlorine or bro-

mine with benzene in the presence of an iron(III) chloride catalyst or aluminium

chloride catalyst.

As the chlorine molecule approaches the benzene ring it becomes polarised by the

delocalised electron cloud. The positiviely charged end of the chlorine molecule is

now an electrophile and attacks the benzene ring.

Bromination would occur in a similar manner however instead of the chloride catalyst

either Iron(II) bromide or aluminium bromide would be used.

Nitration of benzene

In nitration of benzene the electrophile is the nitronium ion. It is this ion which will

attack the benzene ring to give a nitrobenzenium ion and then nitrobenzene.

concentrated nitric and sulphuric acid which produce the nitronium ion, NO2+

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Alkylation of benzene

Alkylbenzenes can be formed by reacting benzene with a halogenoalkane with alu-

minium chloride acting as a catalyst.

In this reaction, known as a Friedel-Craft's alkylation, the aluminium chloride

generates a positive electrophilic substituting reagent (CH2CH3+).

Sulfonation of benzene

Sulfonation involves the substitution of a hydrogen atom of the benzene ring for the

sulfonic acid group. This is brought about by heating the benzene with a slight excess

of concentrated sufuric acid. The electrophile is the SO3- which even though is

neutral acts as a powerful electrophile.

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Phenol and Phenylamine

Phenol is an aromatic compound with the structure.

The phenol compound contains the hydroxyl group which is common to alcohols how-

ever it is this which changes the properties of the benzene ring.

Phenol acts a a weak acid due to the attached OH group. The electron withdrawing

nature of benzene means that the polarity of the OH group is increased which

causes a very limited ionisation when phenol is dissolved in water.

Phenol is the only alcohol to show acidic properties.

The phenol itself has to lose the hydrogen of the OH group to exhibit the acidic

properties. Though the ionisation is so weak that the phenol does not react with car-

bonates to give off carbon dioxide—as phenol is even weaker than carbonic acid.

Th ability of the negative charge on the phenoxide ion to be shared with the delocal-

ised electrons in the benzene ring makes its formation possible.

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Other aromatic compounds

There are a large number of substituted benzene compounds which are used in

the manufacture of a large number of consumer product. We can also get com-

pounds which contain more than one benzene ring in the structure. ( There is no

need to remember these structures. )

Synthetic Routes

The starting point in planning a synthesis is to examine the final product of the

synthesis. This is called the target compound. By looking at the functional

groups and structure of the target molecule, chemists can work backwards,

through a logical sequence of reactions, until a suitable starting compound is

found. The synthetic route to the target compound can involve many steps and

can take years to achieve the synthesis of the target molecule.

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