Introduction to Organic Chemistry
8 marks · HardOrganic chemists use systematic naming to identify complex molecules. The rules for the type of compound and for branched chains must both be followed for names to be correct.
State the name given to the type of compound with the structure shown below. Explain which features of the structure identify this type.
A branch is added to the compound in part (a) by replacing one of the hydrogen atoms on the second carbon (counting from the left) with an ethyl group (–CH2CH3).
i) Draw the full displayed formula for the new compound.
ii) Using the rules for branched chains and the type of compound, write the full systematic (IUPAC) name for your structure.
Draw the displayed formula for the molecule described.
Give the IUPAC name for the compound you have drawn, following the rules for branched chains.
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.
Show answer
(a) alkane; features: only single bonds between carbon atoms, each carbon forms four single bonds, only carbon and hydrogen present.
(b(i)) Displayed formula showing a 5-carbon chain (longest), with an ethyl branch attached to carbon 2, and all correct hydrogens; see solution for diagram features. · must show Longest chain identified (pentane backbone), Ethyl group shown on carbon 2, All carbons with four bonds
(b(ii)) 2-ethylpentane
Common mistakes: Incorrectly placing the ethyl group on the wrong carbon; Not counting the longest chain; Omitting the position number for the branch
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States the correct type: alkane
B2 (a) · 1 mark · Mentions only single bonds present / all C–C bonds are single
B3 (a) · 1 mark · States that only carbon and hydrogen atoms are present
M1 (b(i)) · 1 mark · Longest chain correctly identified in new structure (5 carbons/pentane backbone) (follow-through)
A1 (b(i)) · 1 mark · Correctly shows an ethyl group attached to carbon 2 and hydrogens adjusted so all carbons have four bonds
M2 (b(ii)) · 1 mark · Correctly numbers the chain so the ethyl group is at carbon 2 (follow-through)
A2 (b(ii)) · 1 mark · Gives the correct IUPAC name: 2-ethylpentane
B4 (b(ii)) · 1 mark · Spelling and format correct: no gaps, numbers and dashes correct
Worked solution
a) The structure shown is:
This is an alkane, because all the carbon–carbon bonds are single bonds, and the structure contains only carbon and hydrogen atoms. Each carbon forms four single bonds.
b) i) To add an ethyl group at carbon 2 (counting from the left), we must: identify the longest chain (now 5 carbons – pentane), and attach the ethyl (–CH2CH3) to carbon 2.
ii) The main chain is pentane and the ethyl group is attached to carbon 2. The correct name is 2-ethylpentane.