Q.IUPAC name of CH3-CH(C2H5)-C(C4H9)(CH3)-CH3, i.e. CH3-C(H)(C2H5)-C(C4H9)(CH3)-CH3 drawn as a 4-carbon backbone CH3-C-C-CH3 in which the second carbon carries H (implicit) and an ethyl (C2H5) substituent, and the third carbon carries a butyl (C4H9) substituent and a methyl (CH3) substituent, is

Every IUPAC name is built from exactly three pieces, always assembled in the same order: prefix + root word + suffix. The root word encodes how many carbons sit in the compound's longest continuous chain (meth- for 1, eth- for 2, prop- for 3, but- for 4, pent- for 5, hex- for 6, hept- for 7, oct- for 8, non- for 9, dec- for 10, and so on up to hect- for 100). The prefix names whatever substituent groups are attached to that chain -- the halogens (fluoro-, chloro-, bromo-, iodo-), nitro-, alkoxy groups (methoxy-, ethoxy-), and the branched alkyl substituents (methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl -- $CH_3CH(CH_3)CH_2-$ --, tert-butyl -- $(CH_3)_3C-$ --, n-pentyl, neopentyl). The suffix comes in two layers: a primary suffix recording the chain's saturation (-ane for all-single-bonded, -ene/-diene/-triene for one/two/three C=C bonds, -yne/-diyne for one/two C≡C bonds), and, layered on top of it by dropping its terminal 'e', a secondary suffix recording the actual functional group (-ol for alcohols, -al for aldehydes, -one for ketones, -oic acid for carboxylic acids, -oate for esters, -oyl chloride for acid chlorides, -amide for amides, -amine for amines, -nitrile for nitriles, -sulphonic acid for sulphonic acids, -thiol for thiols).

The naming procedure runs in five steps. (1) Pick the longest chain -- but if the compound carries a recognised functional group, the chosen chain MUST include that group's carbon even if a longer chain exists that skips it; with more than one functional group present, the chain must include whichever sits highest in this precedence order: $-COOH > -SO_3H > -COOR/-COX > -CONH_2 > -CN > -CHO > -CO- > -OH > -SH/-NH_2 > -C{\equiv}C- > -C{=}C- > -C{-}C- > -O- > -X > -NO_2$; with the SAME group appearing more than once, the chain must include the maximum possible count of it. Ties in chain length are broken by preferring the chain with more substituents. (2) Number the chain from whichever end gives the lowest locant, in priority order: first to the principal (suffix) functional group, then to unsaturation, and only then to substituents as a set; a genuine remaining tie goes to whichever substituent is alphabetically first. (3) Name every substituent. (4) List multiple substituents alphabetically before the root word (ignoring di-/tri-/tetra- for the alphabetising, though they stay in the written name). (5) Assemble: prefix + root + primary suffix + secondary suffix.

Formatting conventions finish the system: the name is written as one word (except salts/acids/derivatives); commas separate adjacent locants, hyphens separate a locant from a letter (2,2-Dimethyl-3-hexene); italicised descriptors (cis-, trans-, meso-) are hyphen-joined and ignored when alphabetising; multiplying prefixes (di-, tri-, tetra-) are plain (no italics/extra hyphen) and likewise ignored when alphabetising. …