Alcohols – Reactions of ethanol

Combustion :

Ethanol (along with all organic compounds) burns in excess oxygen to give carbon dioxide and water as the only products.

CH3CH2OH + 3O2(g)  2CO2(g) + 3H2O(g)

N.B.: Less oxygen is needed to ensure complete combustion of an alcohol, than a corresponding alkane, because it contains an oxygen atom already.

(2) With sodium metal :

Just as with water, sodium metal will react with ethanol, though not nearly as violently. The products are hydrogen gas (exactly the same as with water) and sodium ethoxide (different to the reaction with water ),

2CH3CH2OH + 2Na(s)  2CH3CH2ONa + H2(g)

The hydrogen gas produced does not ignite, as the enthalpy change of reaction is not high enough. This is because the oxygen atom in water has two hydrogen atoms attached to it and only one in ethanol.

(3) Oxidation :

Refluxing ethanol with oxidising agents such as acidified manganate(VII)(aq) and acidified dichromate(VI)(aq) ions changes it into ethanoic acid.

CH3CH2OH + [O]  CH3COOH

Purple manganate(VII) ions (MnO4-) turn colourless (Mn2+)

and orange dichromate(VI) ions (Cr2O72-) turn green (Cr3+).

(4) Halogenation:

Whilst it is easy to hydrolyse haloalkanes into alcohols (see the haloalkanes page), the reverse reaction requires quite extreme conditions.

A concentrated mixture of halide ions and acid are used (for example, sodium chloride in concentrated sulphuric acid) along with heat, to ensure that the -OH group leaves the alcohol (as water) and the carbon atom will accept a Cl- ion.

CH3CH2OH + Cl- + H+  CH3CH2Cl + H2O

There is an old test involving the halogenation of an alcohol to determine whether an alcohol is primary, secondary or tertiary. A solution of zinc chloride in hydrochloric acid is added to the alcohol. A primary alcohol will be the slowest to show a reaction and the tertiary will be the fastest.

(5) Dehydration :

This process is the removal of the elements of water (2 H atoms and 1 O atom) from ethanol leaving ethene. It is accomplished by refluxing ethanol with a catalyst of concentrated sulphuric or phosphoric acid, or by passing ethanol vapour over heated aluminium oxide.

CH3CH2OH  CH2=CH2 + H2O

(6) Esterification :

An ester is formed by reacting an alcohol with a carboxylic acid. It is a similar type of reaction to the neutralisation of an acid with a base. A catalyst of concentrated sulphuric acid removes the water produced and helps to push the equilibrium towards the products side of the equation.

CH3CH2OH + CH3COOH  CH3COOCH2CH3 + H2O

(7) Reaction summary chart :

Alcohols – Iodoform reaction

The iodoform test is a test for the existence of the CH3-CO- group in a molecule. This could be part of an alcohol (C-O single bond) or part of a carbonyl compound (C=O double bond).

The reagents use are aqueous sodium hydroxide and iodine crystals. The first stage, in the use of this test with alcohols, is the oxidation of the alcohol group to a carbonyl group,

The hydrogen atoms on the methyl group are slightly acidic and can be removed with sodium hydroxide (stage 1 below).

The carbanion formed then react with iodine molecules to give an iodide ion and an organic iodo compound (stage 2 below).

This substitution continues until a triiodo group has been formed (stage 3 below) by repeated use of sodium hydroxide and iodine.

Another hydroxide ion can then attack the carbonyl carbon atom, giving a carboxylic acid and releasing the CI3 group which abstracts a proton from a water molecule to give CHI3 (called triiodomethane or iodoform) (stage 4 below).

Triiodomethane is a straw yellow solid, insoluble in water. This test works for ethanol and all 2-substituted secondary alcohols,

ethanol :
2-substituted alcohols :
	R = any organic group

Alcohols – Oxidation differences

Most of the reactions of alcohols – combustion, halogenation, dehydration and esterification – do not depend on the nature of the alcohol. The one exception is oxidation.

Oxidation can be referred to as the removal of an atom of hydrogen from a molecule. The extent of oxidation of an alcohol will therefore depend on the number of hydrogen atoms that can be removed from the carbon atom attached to the hydroxyl group,

1° alcohol –

2 hydrogen atoms are bonded to the carbon atom therefore two oxidation steps are possible i.e.

alcohol  aldehyde  carboxylic acid.

2° alcohol –

1 hydrogen atom is bonded to the carbon atom therefore only one oxidation step is possible i.e.

alcohol  ketone.

3° alcohol –

0 hydrogen atoms are bonded to the carbon atom therefore

no oxidation steps are possible.

Alcohols – I.R. Spectroscopy

The spectrum of light is made up of a vast range of wavelengths. Only certain wavelengths are visible to human eyes, those from about 300 nm to 900 nm (1 nm = 1×10-9 m).

Wavelengths shorter than this range have high energy, such as ultra-violet light, x-rays and g-rays. These parts of the spectrum have enough energy to break covalent bonds in molecules and can cause cancer in humans, e.g. skin cancer from strong sunlight.

Wavelengths longer than the above range have low energy, such as infra-red light, microwaves and radio waves. These parts of the spectrum are used in both short-range and long-range communications.

Additionally, infra-red light causes covalent bonds to vibrate, not enough to break the bonds, just to cause the atoms that make up the bond to shift position slightly with respect to one-another.

In infra-red spectroscopy, an organic sample is subjected to wavelengths of light from 4000 cm-1 to 400 cm-1 (a cm-1 is known as a wavenumber, or inverse-centimeter in the US and equal to 1/cm).

Covalent bonds that are formed from two different atoms will absorb particular wavelengths of that infra-red light, and the absence of these wavelengths is detected. This produces graphs such as those shown below.

The particular covalent bonds that have to be identified at AS level are shown below,

an alcohol, e.g. ethanol – an absorption at 3200 to 3600 cm-1

a carbonyl compound, e.g. propanal – an absorption at 1650 to 1720 cm-1

a carboxylic acid, e.g. ethanoic acid – an absorption at 2500 to 3300 cm-1 (-OH group) and 1700 to 1720 cm-1 (C=O group

old notes: (Just in case)

Alcohols are saturated molecules containing an –OH group.

Since the H in the bond O-H can produce hydrogen bonds they have high boiling points. This is also the reason why water dissolves most alcohols. The solubility in water decreases with the length of the carbon chain.

Ethanol, C₂H₅OH, is the most important of the alcohols.

It is used as a solvent and fuel. It is also present almost pure in alcoholic beverages

Making ethanol

Ethanol can be industrially produced by

1. •  
   • hydration of ethene.
   • fermentation of sugars

The method used depends on the desired purity of the ethanol and the availability of the different raw materials in the country where it is manufactured.

i) fermentation of sugars

At 35 – 55 ^oC, sugars such as glucose can be fermented by yeast and turned into ethanol and carbon dioxide. This process must be carried out in the absence of air:

C₆H₁₂O₆  2C₂H₅OH + 2CO₂

This process has a number of advantages:

–          it is a low-technology process, which means it can be used anywhere

–          it does not use much energy

–          it uses sugar cane as a raw material, which is a renewable resource

There are, however, a few disadvantages associated with this process:

–          it is a batch process, which means that once the reaction has finished the vessel needs to emptied before the reaction can be started again

–          it is a relatively slow process

–          it produces fairly impure ethanol

Ethanol for human consumption is manufactured during this process. Some ethanol made in this way is also used as fuels in countries such as Brazil, which have an abundant supply of sugar cane.

ii) hydration of ethene

At 300 ^oC and 60 atmospheres with a concentrated H₃PO₄ catalyst, H₂O can be added to ethene to make ethanol:

C₂H₄ + H₂O  C₂H₅OH

This process has a number of advantages:

–          it is a relatively fast process

–          it is a continuous flow process, which means that ethene can be entered into the vessel continuously and the reaction never has to be stopped

–          it produces pure ethanol

There are also a number of disadvantages associated with this process:

–          it requires fairly high technology

–          it uses a lot of energy

–          the ethene comes from crude oil, which is a non-renewable resource

Ethanol for use in industry is manufactured during this process.

2. Ethanol as a fuel

Ethanol is a useful fuel; it burns with a clean flame and is increasingly used in cars:

C₂H₆O(l) + 3O₂(g)  3CO₂(g) + 3H₂O(g)

If the ethanol used has been produced by fermentation, then it can be classified as a renewable fuel. A fuel derived or produced from renewable biological sources is known as a biofuel.

Biofuels are carbon-neutral. Although they release carbon dioxide when they are burned, they come from plant sources which absorb carbon dioxide from the atmosphere during photosynthesis while they are growing. Thus there are no net emissions of carbon dioxide during the process from growing to combustion.

3. Primary, secondary and tertiary alcohols

Alcohols can be divided into three classes: primary, secondary and tertiary.

Primary alcohols are those in which the carbon attached to the OH is attached to 0 or 1 other carbon atom. In other words, they are molecules in which the functional group is at the end of the chain.

Eg propan-1-ol

Secondary alcohols are those in which the carbon attached to the OH is attached to 2 other carbon atoms. In other words, they are molecules in which the functional group is not at the end of the chain.

Eg propan-2-ol

Tertiary alcohols are those in which the carbon atom attached to the OH is attached to 3 other carbon atoms. In other words, they are molecules in which the functional group is attached to a carbon which also has a branch attached to it.

Eg 2-methypropan-2-ol

3.         Reactions of alcohols

Alcohol molecules are saturated and polar, containing a d+ve carbon. Thus alcohols tend to undergo nucleophilic substitution reactions.

The OH can combine with an adjacent H atom to form a stable H₂O molecule. Thus alcohols can also undergo elimination reactions.

Alcohols can lose hydrogen and undergo a variety of oxidation reactions.

a)         elimination reactions

Like halogenoalkanes, alcohols can undergo elimination to give alkenes. Since alcohols lose water when they undergo elimination, the reaction is also called dehydration.

The ethanol should be heated and passed over a catalyst (pumice can be used).It can also be refluxed at 180^oC with concentrated sulphuric acid.

Alkenes produced in this way can be polymerised. This method therefore allows polymers to be produced without using crude oil (assuming that the original ethanol was produced by fermentation).

The dehydration of alcohols is favoured by acidic conditions, as the -OH group becomes protonated by H⁺ ions which produces a water molecule which then leaves. The acid acts as a catalyst. The detailed mechanism is not required.

The H which is lost comes from a carbon atom which is adjacent to the carbon atom attached to the OH group. In some cases, this can lead to more than one product.

Eg butan-2-ol:

When butan-2-ol undergoes elimination, two different products can be formed depending on which H atom is lost:

Butan-2-ol  à but-2-ene:

Butan-2-ol à but-1-ene

NB Alcohols which have no H atoms on the C atom adjacent to the OH group cannot undergo elimination:

b)         oxidation reactions

Oxidation in organic chemistry can be regarded as the addition of oxygen or the removal of hydrogen. As the full equations are quite complex, the oxidising agent is represented by the symbol [O].

a)         mild oxidation of primary and secondary alcohols

If a primary alcohol is mixed with an oxidising agent, two hydrogen atoms can be removed and an aldehyde will be formed:

Eg CH₃CH₂OH + [O]  CH₃CHO + H₂O

An aldehyde is a molecule containing the following group:

If a secondary alcohol is mixed with an oxidising agent, two hydrogen atoms can be removed and a ketone will be formed:

Eg CH₃CH(OH)CH₃ + [O]  CH₃COCH₃ + H₂O

A ketone is a molecule containing the following group:

Tertiary alcohols are not readily oxidised since they do not have available H atoms to give up.

Aldehydes and ketones are collectively known as carbonyls and can be represented by the general formula C_nH_2nO

In aldehydes, one of the R groups is a H atom. In ketones, neither of the R groups is a H atom.

b)       further oxidation of aldehydes

If an aldehyde is mixed with an oxidising agent, an oxygen atom can be added to the group and a carboxylic acid will be formed:

Eg CH₃CHO + [O] à CH₃COOH

A carboxylic acid is a molecule containing the following group

Ketones cannot be oxidised into carboxylic acids since there is no C-H bond into which an oxygen atom can be inserted.

c)      reagents and conditions for oxidation

The oxidising agent most widely used in organic chemistry is potassium dichromate (K₂Cr₂O₇) in dilute sulphuric acid (H₂SO₄).

Cr₂O₇^2-(aq) + 14H⁺(aq) + 6e à 2Cr³⁺(aq) + 7H₂O(l)

The Cr₂O₇^2-(aq) ion is orange and the Cr³⁺ ion is green. Thus this reduction process is accompanied by a colour change from orange to green.

If primary alcohols are oxidised, it is possible to form both aldehydes and carboxylic acids. The major product will depend on the conditions used.

Carbonyls are more volatile than alcohols and carboxylic acids, since there is no hydrogen bonding between aldehyde molecules. Thus if a distillation apparatus is used, the volatile aldehyde can be distilled off as it is formed. If reflux apparatus is

used, the aldehyde remains in the reaction vessel and is converted into the carboxylic acid.

Thus distillation apparatus should be used to make carbonyls and reflux apparatus should be used to make carboxylic acids. Heat and an excess of the oxidising agent also improve the yield of carboxylic acid.

Secondary alcohols are oxidised to make ketones only.  The distillation apparatus is still favoured since the ketone is volatile so can be distilled off as it is formed.

Thus the oxidation reactions of alcohols and aldehydes can be summarised as follows:

R-CH₂OH + [O]  R-CHO + H₂O (primary alcohol à aldehyde)

K₂Cr₂O₇, H₂SO₄, mild conditions, distillation.

R-CH₂OH + 2[O]  R-COOH + H₂O (primary alcohol à carboxylic acid)

Excess K₂Cr₂O₇, H₂SO₄, heat, reflux

R-CHO + [O]  R-COOH (aldehyde à carboxylic acid)

Excess K₂Cr₂O₇, H₂SO₄, heat, reflux

R₁-CH(OH)-R₂ + [O]  R₁-CO-R₂ + H₂O (secondary alcohol à ketone)

K₂Cr₂O₇, H₂SO₄, heat, distillation

Summary of oxidation reactions of alcohols and carbonyls

1. Tests to distinguish between aldehydes and ketones

a)      Tollen’s reagent

Aldehydes and ketones can be distinguished by their reaction with ammoniacal silver nitrate (known as Tollen’s reagent). Aledehydes are reducing agents since they can be oxidised to carboxylic acids, but ketones are not reducing agents. Ammoniacal silver nitrate, or Tollen’s reagent, is an oxidising agent and will react with aldehydes on boiling:

In the presence of aldehydes, the colourless Ag⁺ ions are reduced to metallic silver, which forms on the surface of the test tube.

The presence of a “silver mirror” indicates that an aldehyde is present.

b)     Fehling’s solution

Aldehydes and ketones can also be distinguished by their reaction with Fehling’s solution. Fehling’s solution is a complex solution containing Cu²⁺ ions. Aldehydes are reducing agents but ketones are not. Cu²⁺ is an oxidising agent and will react with aldehydes on heating.

In the presence of aldehydes, the blue Cu²⁺ is reduced to the red copper (I) oxide, Cu₂O.

The presence of a brick red precipitate of Cu₂O indicates that an aldehyde is present.

1. Summary of reactions of alcohols and carbonyls

http://www.chemistryrules.me.uk/candr/alcohols.htm