Chemistry · General Principles Of Organic Chemistry · NEET
The longest chain is not always the one drawn horizontally. Trace every possible path of connected carbons, including paths that turn corners, and pick the one with the most carbon atoms. That chain becomes the parent alkane (its carbon count gives meth, eth, prop, but, pent, hex... -ane). Any carbon not on this chain is a branch (substituent). A common NEET trap is choosing the straight-drawn chain when a longer bent chain exists.
Number the parent chain from the end that gives the LOWEST set of locants to the substituents. Compare the two possible numbering directions at the first point of difference: the direction giving the smaller number there wins. Example: branch reachable as position 3 from the right but position 5 from the left, choose the right so the locant is 3.
No. Multiplying prefixes (di, tri, tetra) and separating hyphens are IGNORED when you alphabetise. You alphabetise by the base substituent name only. So 'ethyl' (e) comes before 'methyl' (m), and 'diethyl' is still filed under 'e'. But structural prefixes that are part of the name, like iso in isopropyl, ARE counted.
Yes. e comes before m in the alphabet, so ethyl is cited before methyl regardless of their locant numbers. This is exactly what NEET 2022 tested: the answer 3-ethyl-5-methylheptane lists ethyl first even though its locant (3) and methyl's locant (5) could tempt you to reorder. Name order follows the alphabet, not the position numbers.
If the locant sets are identical from both ends (a tie), assign the lowest locant to the substituent that comes first alphabetically. This is the tie-breaker rule and NTA loves testing it with symmetric-looking molecules.
The correct IUPAC name of a branched alkane whose longest continuous chain is 7 carbons, with an ethyl group and a methyl group as branches, is:
A compound with molecular formula C6H14 has two tertiary carbons. Its IUPAC name is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1) Select the longest continuous carbon chain as the parent. 2) Number the chain to give the lowest set of locants to substituents. 3) Name and number each substituent, cite them alphabetically as prefixes, and use di/tri/tetra for repeated ones. 4) Combine into: locant-prefix + parent alkane name.
Substituents are listed in alphabetical order by their base name. 'Ethyl' starts with e, 'methyl' with m, so ethyl comes first, independent of which carbon each is attached to.
No. Di, tri, tetra are ignored when alphabetising. You alphabetise by the substituent's base name (e.g., 'dimethyl' is filed under m for methyl).
When two numbering directions are possible, choose the one giving the lowest set of locants at the first point of difference. If both give the same set, give the lowest number to the substituent first in alphabetical order.
Usually one direct question (4 marks) in most years, often a name-the-structure or match-the-structure MCQ. It is a high-return, low-effort topic once the three-step rule is memorised.
