CARBONYL COMPOUNDS TEST

1. Insects use certain small organic molecules to communicate with their fellows by releasing these molecules in small amounts. The structural formula of one such molecule is shown below. It is used to signal ‘alarm’ in ants.

CH₃CH₂CHCOCH₂CH₃

½

CH₃

(a) (i) Give the systematic name for the structure above

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(ii) The molecule contains a chiral centre. Mark this centre with an asterisk (*) in the formula above and explain what is meant by a chiral centre.

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(iii) Draw the two stereoisomers (optical isomers) of this molecule.

(b) Another alarm chemical found in ants is hex-2-enal:

CH₃CH₂CH=CHCHO

(i) How would you show, by a chemical test, that hex-2-enal is unsaturated?

Give an equation for the reaction involved.

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(ii) Hex-2-enal can exist as two geometric isomers. Drawn the structure of the isomers and label the cis structure.

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(c) Both of these alarm pheromones, as they are known, have relatively small molecules.

(i) What advantage, to the insect, is there in the small molecular mass of the alarm pheromones?

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(ii) The small molecular size is increased when the compounds are reacted with 2,4-dinitrophenylhydrazine.

Write an equation for the reaction of this reagent with one of the pheromones.

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(iii) What is the usual purpose of forming 2,4-dinitrophenylhydrazones from aldehydes or ketones?

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(iv) What would be observed when 2,4-dinitrophenylhydrazine and propanal are mixed?

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(d) Propanal is an aldehyde. How could Fehling's solution be used to confirm this?

(Include in your answer what you would expect to see).

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2 Acetone, (CH₃)₂CO, can be prepared industrially from propan-2-ol by oxidation with air at 600 ^oC in the presence of copper.

(a)(i) Give the IUPAC name for acetone.

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(ii) What is the purpose of copper in the reaction?

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(iii) Name an oxidising agent that is normally used to oxidise an alcohol in the laboratory.

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(iv) What advantage is there in using air as the oxidant in the industrial process7

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(v) Name the product formed, and give its structure, when propan-1-ol is used as the starting material.

Name

Structure

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(b) The industrial oxidation of propan-2-ol can be demonstrated in the laboratory using the apparatus below

The acetone is passed into a solution of 2,4-dinitrophenylhydrazine (2,4-DNPH).

The yield of the reaction is calculated from the mass of hydrazone formed.

(i) What observation would tell you that a reaction was taking place in the 2,4-DNPH solution ?

(ii) How would you determine the mass of propan-2-ol used in the reaction ?

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(c) The reaction of acetone with 2,4-DNPH is shown by the equation below.

(i) Calculate the molar masses of acetone and its hydrazone.

(Relative atomic masses: H=1, C=12, N=14, O=16)

Acetone:

Molecular formula

Molar mass

Hydrazone:

Molecular formula

Molar mass

(ii) If 1.19 g of the hydrazone is produced what mass of acetone was needed to form it?

(iii) Calculate the percentage yield of acetone if 0.40 g of propan-2-ol was used.

[8]

3. A species of millipede found in the U.S.A. stores the cyanohydrin, X, as a defense material. The millipede can break down the cyanohydrin to give benzaldehyde and hydrogen cyanide which act as deterrents to attackers. The reaction is reversible.

(a) The cyanohydrin is a polar molecule.

(i) What is meant by the term polar?

(ii) Redraw the cyanohydrin structure showing clearly the polarity of the –OH group.

(iii) Explain why you think the cyanohydrin might be soluble in both water and benzene.

(b)