MC test: Alcohols

Multiple choice test on 10.2(2) Alcohols

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet).

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

The general equation for the complete combustion of alcohols can be written as

CxHyO + aO2 → xCO2 + bH2O

Which gives the correct values for a and b?

For every mole of CxHyO combusted y/2 mol of water will be formed. The amount of oxygen atoms required = 2x + y/2 −1 so the amount of oxygen molecules  = x + y/4 − 0.5 and b = y/2

 

Which is a secondary alcohol?

A secondary alcohol has one hydrogen atom and two R− groups bonded to the carbon atom containing the hydroxyl group.

 

Which alcohol cannot be oxidized by a warm acidified solution of potassium dichromate(VI)?

Tertiary alcohols cannot be oxidized by acidified dichromate(VI) ions. 2-methylpropan-1-ol and propan-1-ol are primary alcohols and butan-2-ol is a secondary alcohol.

 

What organic product will be formed when pentan-2-ol is oxidized by an acidified solution of potassium manganate(VII).

Secondary alcohols are oxidized to ketones. The carbonyl group of the ketone will be in the same position in the carbon chain as the −OH group of the alcohol.

 

Ethanol can be oxidized first to ethanal and then to ethanoic acid when it is heated under reflux with an acidified solution of potassium dichromate(VI). What method can be used to obtain ethanal from this reaction before it reacts further to form ethanoic acid?

Ethanal boils at 21 oC and ethanol and ethanoic acid boil at 78 oC and 118 oC respectively. By removing the ethanol by distillation as soon as it is formed it cannot react further as it is no longer in contact with the oxidizing agent.

 

Which is the correct equation for the oxidation of ethanol to ethanoic acid?

The half-equations are C2H5OH + H2O → CH3COOH + 4H+ + 4e− and Cr2O72− + 14H+ + 6e− → 2Cr3+ + 7H2O
The first equation needs to be multiplied by 3 and the second by 2 to give 12 electrons in both equations then the two equations are added together.

 

What colour change will be observed when an acidified solution of potassium dichromate(VI) is added to an excess of a secondary alcohol and the mixture is warmed?

The Cr2O72−(aq) ion is orange and it is reduced to the green Cr3+(aq) ion.

 

Which can undergo oxidation and still retain the same number of carbon atoms in the longest chain?

I. primary alcohols

II. aldehydes

III. secondary alcohols

Primary alcohols can be oxidized to aldehydes and carboxylic acids, aldehydes can be oxidized to carboxylic acids and secondary alcohols can be oxidized to ketones. Tertiary alcohols can only be oxidized by combustion which breaks down the carbon chain.

 

Which are correct descriptions of the type of reaction that occurs when an alcohol and a carboxylic acid react together in the presence of an acid catalyst?

I. esterification

II. condensation

III. nucleophilic substitution

The product is an ester formed by a condensation reaction in which two smaller molecules react to form a larger molecule with the elimination of a small molecule such as water. After the first step in the reaction (protonation of the carboxylic acid by the acid catalyst) the lone pair on the O atom of the alcohol acts as the nucleophile and the H of the alcohol is eventually substituted by –COR, so it can also be described as a nucleophilic substitution reaction.

 

Which is the correct equation for the formation of propyl methanoate?

Propyl methanoate,  is formed by condensing methanoic acid with propan-1-ol. The reaction does not go to completion as it reaches a position of equilibrium.

 

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