Chemistry · Hydrocarbons · NEET
Do not just count the straight horizontal line. The longest chain can bend and turn corners. Trace every possible path through the carbons and count the carbons in each path. Pick the path with the MOST carbons, even if it goes up or down. That longest path is your parent chain. If two paths have the same length, pick the one with more branches on it.
Number from the end that gives the branches (substituents) the LOWEST set of numbers, called locants. Compare the two possible numbering directions. Add up or compare the branch positions and choose the smaller set. Example: if one direction gives branches at 3 and 5, and the other gives 3 and 5 too, then at the first point of difference pick the lower one. This is why 3-ethyl-5-methylheptane is correct, not 5-ethyl-3-methylheptane.
Three parts build the name. The ROOT word tells how many carbons are in the parent chain (meth=1, eth=2, prop=3, but=4, pent=5, hex=6, hept=7). The SUFFIX tells the bond type: -ane (single), -ene (double), -yne (triple). The PREFIX tells the branches (methyl, ethyl) with their position numbers. So 2-methylpentane = pent (5 C) + ane (all single bonds) + methyl branch at carbon 2.
Use them when the SAME branch appears more than once. Two methyl groups = dimethyl, three = trimethyl. You must still write a position number for EACH one, separated by commas. Example: 2,3-dimethylbutane has two methyl groups, one at carbon 2 and one at carbon 3. Note: di/tri are NOT counted for alphabetical order.
The double bond (-ene) or triple bond (-yne) must get the lowest possible number, and it beats branches in priority. Number from the end nearest the multiple bond. Put the number of the FIRST carbon of the double bond just before -ene or -yne. Example: CH2=CH-CH2-CH3 is but-1-ene, because the double bond starts at carbon 1.
When there are different branches, write them in alphabetical order of the branch name. Ethyl comes before methyl (e before m). Ignore the di/tri multiplier when alphabetising. So in 3-ethyl-5-methylheptane, ethyl is written first because 'e' comes before 'm', not because of its number.
The correct IUPAC name of the compound (an alkane with an ethyl group and a methyl group on a heptane chain) 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) Find the longest carbon chain (parent). 2) Number the chain so branches get the lowest numbers. 3) Identify and name the branches (substituents). 4) Use di/tri/tetra for repeated branches and write them alphabetically. 5) Combine the numbers, branch names and parent name, using the correct ending (-ane, -ene, -yne).
Molecules are drawn on paper as zig-zags, so the truly longest chain may turn a corner. You must trace all paths and count carbons. Missing a longer chain is the most common mistake and gives the wrong parent name.
The double or triple bond gets higher priority than a branch. Number to give the multiple bond the lowest number first; only if there is no multiple bond do branches decide the numbering.
NEET regularly asks you to pick the correct IUPAC name of a drawn structure, or to match a name to a structure with a certain feature (like tertiary carbons). Getting the longest chain, lowest locants and alphabetical order right earns easy, guaranteed marks.
Alphabetical order controls the order in which branch names are written, independent of their locant numbers. Ethyl (e) is written before methyl (m) even though its position number may be smaller or larger.
