Lowest Locant Rule and Alphabetical Numbering in IUPAC Names
Chemistry · General Principles Of Organic Chemistry · NEET
The lowest locant rule says: number the main carbon chain from the end that gives the lowest set of position numbers (locants) to the substituents. When two names are still possible, you then list substituents in alphabetical order, and if there is still a tie, the substituent that comes first alphabetically gets the lower number. Memory hook: "First lowest numbers, then A-B-C order" - numbers win first, alphabet breaks the tie.
A 7-carbon (heptane) chain with ethyl and methyl branches: numbering from either end gives the same locant set {3,5}, so the alphabetical tie-breaker gives ethyl the lower number, producing 3-ethyl-5-methylheptane (the NEET 2022 answer).
Your doubts, answered
How do I decide which end of the chain to number from?
Number from both ends and write down the set of locants (position numbers) for the substituents each way. Compare the two sets term by term, from smallest to largest. The end that gives the smaller number at the first point of difference wins. Example: locants {2,5,6} vs {3,4,5} - compare 2 vs 3, and 2 is smaller, so the first set wins. This is why you never just add the numbers.
What exactly does 'lowest set of locants' mean?
Line up the position numbers in increasing order for each direction of numbering. Then compare them one by one. The 'lowest set' is the one that is smaller at the first place where the two lists differ - not the one with the smaller total. Example: {2,3,7} beats {2,4,5} because at the second number, 3 is less than 4, even though 2+3+7=12 is larger than 2+4+5=11.
Does alphabetical order decide the numbering, or the writing order?
Both, but at different times. Rule 1 (lowest locants) decides the numbering first. Alphabetical order is used for two things: (1) it always decides the order you WRITE the substituents in the name, and (2) it only decides the NUMBERING when the lowest-locant rule gives a tie. So numbers come first; the alphabet is the tie-breaker for numbering and the fixed rule for writing.
What if both ends give exactly the same set of locants?
Then the lowest locant rule cannot break the tie. Now you give the lowest number to the substituent that comes first in alphabetical order. Example: a chain with a bromo and a chloro that both end up as {2,4} - 'bromo' (b) is before 'chloro' (c), so bromo gets position 2.
When writing the name, is it ethyl before methyl?
Yes. You always write substituents in alphabetical order in the name, so ethyl (e) comes before methyl (m), giving names like 3-ethyl-5-methylheptane. Multiplying prefixes di, tri, tetra are ignored for alphabetising - 'dimethyl' is alphabetised under 'm' for methyl.
Why is it 3-ethyl-5-methylheptane and not 5-ethyl-3-methylheptane?
Both directions give the locant set {3,5}, so the sets are equal and the lowest locant rule ties. The tie-breaker is alphabetical order: ethyl (first alphabetically) must get the lower number, 3. So ethyl is at C-3 and methyl is at C-5, giving 3-ethyl-5-methylheptane. This exact compound was asked in NEET 2022.
⚠️ The NEET trap ✗ Choosing the numbering direction by adding the locants and picking the smaller sum (e.g. picking {2,4,5}=11 over {2,3,7}=12). ✓ Compare the sorted locant sets term by term; pick the one lower at the first point of difference. {2,3,7} wins because 3 < 4 at the second position, regardless of the total. 🧠 NEET tests 'lowest SET', not 'lowest SUM'. Line the numbers up and compare left to right.
Real NEET questions
NEET 2022
The correct IUPAC name of the compound shown (a heptane chain bearing an ethyl group and a methyl group) is:
A · 3-ethyl-5-methylheptane ✓
B · 2,4-diethylhexane
C · 5-ethyl-3-methylheptane
D · 3,5-diethylhexane
Solution: The longest continuous chain is 7 carbons (heptane), ruling out the hexane options B and D. Both directions of numbering give the same locant set {3,5}, so the lowest locant rule ties. The alphabetical tie-breaker then gives the lower number to ethyl (before methyl), so ethyl is at C-3 and methyl at C-5. Substituents are also written alphabetically, giving 3-ethyl-5-methylheptane (option A). Option C is wrong because it puts ethyl at the higher locant.
NEET 2022
The correct IUPAC name of CH3-CH(Cl)-CH(CH3)-CH(OH)-CH2-CH2-Br is:
A · 1-bromo-5-chloro-4-methylhexan-3-ol ✓
B · 6-bromo-2-chloro-4-methylhexan-4-ol
C · 1-bromo-4-chloro-5-chlorohexan-3-ol
D · 6-bromo-4-chloro-4-chlorohexan-4-ol
Solution: The principal chain is 6 carbons (hexane) and -OH is the principal characteristic group, so it gets the lowest possible locant. Numbering from the Br end places OH at C-3; from the Cl end it would place OH at C-4. Since OH must get the lower number, we number from the Br end: Br at C-1, Cl at C-5, methyl at C-4, OH at C-3. Written with substituents in alphabetical order this is 1-bromo-5-chloro-4-methylhexan-3-ol (option A).
Solved General Principles Of Organic Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Number the parent chain from the end that gives the lowest set of locants to the substituents, comparing the position numbers term by term.
Which comes first, lowest locant or alphabetical order?
Lowest locant comes first for numbering. Alphabetical order only decides the numbering if the locant sets tie, but it always decides the order substituents are written.
Do we compare the sum of locants or the set of locants?
The set, not the sum. Sort each set and compare at the first point of difference; the smaller number there wins.
Are di, tri, tetra counted in alphabetical order?
No. Multiplying prefixes (di, tri, tetra) are ignored while alphabetising. 'Dimethyl' is alphabetised under 'm' for methyl, but 'iso' and 'cyclo' are counted.
Does the principal functional group affect numbering?
Yes. The principal characteristic group (like -OH, -CHO, -COOH) gets the lowest locant first; only after that do the lowest-locant and alphabetical rules apply to the remaining substituents.
Why does NEET like this topic?
It is a quick, rule-based question that separates careful students from guessers. One numbering mistake changes the whole name, so it is an easy mark if you apply the rules in the right order.
