Introduction to Organic Chemistry
7 marks · HardThis question is about both the coding for the type of organic compound and the coding for side (branched) chains.
State what is meant by an organic compound, and name two main classes of organic compounds.
The compound 3-ethylpentane has a branched carbon chain.
Draw the skeletal formula for 3-ethylpentane.
Explain how the name '3-ethylpentane' shows information about:
the number of carbons in the main chain,
the type of compound,
the side chain and its position.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) An organic compound is a compound containing carbon atoms, usually bonded to hydrogen, and often also containing other elements. Main classes include hydrocarbons (alkanes, alkenes, alkynes) and alcohols.
(b) Correct skeletal formula for 3-ethylpentane: five-carbon chain with an ethyl group at carbon 3. · must show five carbon main chain, no double bonds, ethyl branch at position 3
(c(i)) 'Pent' shows the main chain has 5 carbons.
(c(ii)) 'ane' indicates it is an alkane (all single bonds).
(c(iii)) '3-ethyl' means an ethyl group (two-carbon chain) is attached to the third carbon.
Must show: skeletal formula for 3-ethylpentane
Common mistakes: Labelling the main chain incorrectly; Placing the ethyl group at position 2 or 4 instead of 3
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correct definition of an organic compound.
B2 (a) · 1 mark · Name any two main classes of organic compounds.
D1 (b) · 2 marks · Correct skeletal formula for 3-ethylpentane, with branch at position 3.
E1 (c(i)) · 1 mark · Main chain length decoded from 'pent' as five carbons.
E2 (c(ii)) · 1 mark · Type of compound from 'ane' as alkane (single bonds).
E3 (c(iii)) · 1 mark · Side chain nature and position from '3-ethyl'.
Worked solution
a) Organic compounds are substances containing carbon atoms, usually bonded to hydrogen, often in chains or rings. Main classes include hydrocarbons (alkanes, alkenes, alkynes) and alcohols.
b) The skeletal formula for 3-ethylpentane:
The main chain is five carbons (A–E, pentane); position 3 (C) attaches to an ethyl group (C–F–).
c) (i) The 'pent' code in the name shows there are five carbon atoms in the longest chain.
(ii) The ending 'ane' shows all carbon-carbon bonds are single, so it is an alkane.
(iii) The prefix '3-ethyl' shows there is a two-carbon (ethyl) branch on the third carbon of the main chain.