Reactions of Alcohols 2021-22

Reactions of Alcohols

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Primary, secondary and tertiary

• A chain of carbon atoms can be represented by R when drawing the structure. This is referred to as an R group.

• R group can either be an alkyl or an aryl.

Primary alcohol

• Primary (1°) alcohols have one R group attached to the carbon to which the OH group is attached.

  
  

Secondary alcohol

• Secondary (2°) alcohols have two R groups attached to the carbon to which the OH group is attached.

  
  

Tertiary alcohol

• Tertiary (3°) alcohols have three R groups attached to the carbon to which the OH group is attached.

  
  

Oxidation of 1° alcohols: aldehydes

• Primary alcohols can be oxidized to aldehydes by an oxidizing agent such as an aqueous solution of acidified potassium dichromate(VI).

• When the symbol equation is written, the oxidizing agent is represented by [O]

Oxidation of 1° alcohols: aldehydes

• Aldehydes contain a carbonyl group (C=O) at the end of the carbon chain, and are named using the suffix –al.

Synthesis of aldehydes

• When aldehydes are prepared by the reaction of a primary alcohol with acidified potassium dichromate(VI), the aldehyde is distilled off and collected, preventing further oxidation.

Oxidation of 1° alcohols: carboxylic acids

• If primary alcohols are reacted with an excess of oxidizing agent and refluxed, they can be oxidized to aldehydes and then oxidized further to carboxylic acids.

Oxidation of 1° alcohols: carboxylic acids

• Carboxylic acids contain a carbonyl group (C=O) at the end of the carbon chain, with a hydroxyl group (OH) attached to the carbonyl carbon.

• Carboxylic acid are named using the suffix –oic acid.

Oxidation of 2° alcohols: ketones

• Secondary alcohols can be oxidized to ketones by an oxidizing agent such as an aqueous solution of acidified potassium dichromate(VI).

Oxidation of 2° alcohols: ketones

• Ketones contain a carbonyl group (C=O) attached to any carbon in the chain except a terminal carbon atom, and are named using the suffix –one.

Tertiary alcohols are resistant to oxidation due to the lack of hydrogen atoms on the carbon atom to which the hydroxyl group is attached.

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Esterification

• Esterification involves refluxing a carboxylic acid and an alcohol with a concentrated sulfuric acid catalyst.

Esterification

• The names of esters have two parts: the first is an alkyl group (from the alcohol) and the second is a carboxylate group (from the carboxylic acid).

• For example, methanol and ethanoic acid react to form methyl ethanoate.

Hydrolysis of Esters

• Esters can be hydrolysed by heating under reflux with

  either an acid or a base.

Hydrolysis of Esters by an Acid

• Refluxing with an acid simply reverses the preparation of the ester from an alcohol and a carboxylic acid.

• The acid catalyses the reaction.

• The reaction is reversible and an equilibrium mixture is established.

• In acid hydrolysis, there are always both reactants (ester + water) and products (carboxylic acid + alcohol) present after the reaction.

Hydrolysis of Esters by a Base

• When an ester is refluxed with an alkali (a

  soluble base), such as aqueous sodium hydroxide, it is fully hydrolysed.

• Unlike acid hydrolysis, this is not a

  reversible reaction, so all the ester present can be broken down by excess alkali.

• An alcohol and the sodium salt of the carboxylic acid are formed.

Making alcohols from aldehydes/ketones

• Aldehydes and ketones can be reduced by a reducing agent, such as sodium borohydride (NaBH₄), to form alcohols.

• When the symbol equation is written, the reducing agent is represented by [H].

Making alcohols from aldehydes/ketones

• Aldehydes are reduced to primary alcohols.

Making alcohols from aldehydes/ketones

• Ketones are reduced to secondary alcohols.

Synthesis of ethene from ethanol 

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Dehydration of ethanol in the lab

• In the lab, dehydration of ethanol can be achieved by passing ethanol over a hot aluminum oxide catalyst. Ethene gas is collected by displacement.

Combustion of alcohols

• Alcohols undergo complete combustion to form carbon dioxide and water.

Combustion of alcohols

• Denatured alcohol is ethanol that has been made toxic and undrinkable by the addition of other chemical additives.

• A traditional additive was methanol, which formedmethylated spirits (meths): 90% ethanol and 10% methanol.

• Denatured alcohol is a useful portable fuel (e.g. for camping stoves) as, unlike LPG, it does not need to be transported in heavy, specialized containers. It is also used as a solvent.

Reaction with Sodium (Na)

Forming halogenoalkanes from alcohols

• Primary, secondary and tertiary alcohols all react with phosphorus(V) chloride to form a chloroalkane.

• Adding solid phosphorous(V) chloride to an alcohol at room temperature produces white HCl fumes, as well as the chloroalkane.

• This reaction can be used as a test for alcohols (the OH group), as well as a method of producing halogenoalkanes.

Multiple Choice

TRUE or FALSE: Ketones can be reduced to primary alcohols using sodium borohydride.

• True

• False

Multiple Choice

TRUE or FALSE: Ethanol can be dehydrated with concentrated sulfuric acid to form ethene.

• True

• False

Multiple Choice

TRUE or FALSE: Tertiary alcohols react with phosphorus (V) chloride to form tertiary halogenoalkanes.

• True

• False

Multiple Choice

TRUE or FALSE: Alcohols undergo complete combustion to form carbon dioxide and water.

• True

• False

Multiple Choice

TRUE or FALSE: Sodium is more reactive with ethanol than with water.

• True

• False

Poll

This lesson is so easy to understand!!!

• I strongly agree.

• I agree.

• I neither agree nor disagree.

• I disagree.

• I strongly disagree.