Alcohols

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Alcohols

-general formula CnH2n+1OH 

Shape:

-The oxygen atom has 2 bonding pairs and 2 lone pairs so angle between bond is about 105 

Classification of alcohols:

Primary: The C with the -OH group has one R group (and so has 2H atoms) 

Secondary: The C with the -OH group has 2 R groups (and so has 1 H atom)

Tertiary: The C with the -OH group has 3 R groups (and so 0 H atoms). These alcohols have OH groups at a branch in the chain 

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Physical properties

The OH group means that H bonding occurs between the molecules, therefore they have higher MP's than alkanes of a similar Mr

The OH group can bond to water molecules but the non-polar hydrocarbon chain cant. This means that short chain alcohols are soluble as the H bonding predominates but long chain ones arent as the hydrocarbon chain predominates. 

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Ethanol production

Making ethanol from crude oil:

-Ethene is produced when crude oil is cracked 

- Ethene can be hydrated with steam and a phosphoric acid catalyst to form ethanol 

Mechanism: 

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Ethanol production

Fermentation: slow and batch process and produces an aqueous solution of ethanol compared to hydration which is fast, continuous and produces pure ethanol 

-Carbohydrates from plants are broken down into sugars by anaerobic respiration and then converted to ethanol by the action of enzymes from yeast

-carbohydrates come from crops like sugar cane and beet

-the rate of reaction is affected by temp. enzyme controlled process so 35 degrees used

- air is kept out to prevent oxidation of ethanol into ethanoic acid 

-once the fermenting solution contains 15% ethanol, enzymes are unable to function 

-ethanol can be distilled from this mixture via fractional distillation 

-you can dehydrate this ethanol to produce ethene which can be used for polymers 

-ethanol made by fermentation is a biofuel 

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reactions of alcohols

Combustion: 

-burn completely to CO2 and H2O if enough oxygen is available. Often used as fuels 

Elimination:

-Water leaves the alcohol- this comes from the OH group and a H atom on the C next to the OH group. This is an example of dehydration 

Dehydration: 

-alcohols can be dehydrated with excess hot conc sulfuric acid or by passing their vapours over heated aluminium oxide. This forms an alkene (and water) 

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Oxidation

alcohols can be oxidised gently and in stages. 

-Primary alcohols can be oxidised to form aldehydes (CHO). Further oxidation forms carboxylic acids (COOH) 

-Potassium dichromate acidified with dilute H2SO4 is the oxidising agent. Orange dichromate ions are reduced to green chromium ions.

-to oxidise to an aldehyde- dilute acid and less potassium dichromate is used than for complete oxidation. The mixture is gently heated and evaporates and then vapourises and distils off into a receiver cooled in ice. The distilling part stops it from oxidising further into a carboxylic acid

- to oxidise to a carboxylic acid- conc sulfuric acid and excess potassium dichromate used. The mixture is then refluxed (so that the ethonol or ethanal vapour condenses and drips back into the flask to be oxidised to acid). You then distil it. 

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Oxidation

-Secondary alcohols are oxidised to ketones (C=O) by acidified dichromate

-Tertiary alcohols arent easily oxidised, because oxidation would require a C-C bond to break rather than C-H

-aldehydes and ketones both have a C=O group- called a carbonyl group. In aldehydes its at the end of the chain and ketones in the body. 

Tests for aldehydes and ketones:

-Tollens (silver mirror) test: Tollens reagent is a gentle oxidising agent- solution of silver nitrate in aqueous ammonia. Oxidises aldehydes but has no affect on ketones. As the aldehyde is oxidised, silver ions are reduced to silver on heating- which coats the inside of the test tube

- The Fehlings test- fehlings reagent is a gentle oxidising agent that contains blue copper ions which will oxidise aldehydes but not ketones. During oxidation on warming, blue solution turns to a brick red precipitate. 

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