Q.Give IUPAC names for the following compounds

Step 1 (structure 1). CH3-CH=CH-CH=CH-C$\equiv$C-CH3 is an eight-carbon chain (octa-) with double bonds starting at C2 and C4 and a triple bond starting at C6 (numbering from the left gives the double bonds the lower locant set when there is a choice, per the rule that ene gets priority over yne for the lowest locant in a tie): octa-2,4-dien-6-yne.

Step 2 (structure 2). CH3-C(C2H5)(CH3)-CH(CH3)-C$\equiv$C-CH3: numbering from the triple-bond end gives it the lower locant (2 rather than 4); the resulting hexyne backbone carries an ethyl and a methyl on C5, plus a methyl on C4: 5-ethyl-4,5-dimethylhex-2-yne.

Step 3 (structure 3). (CH3)3C-C$\equiv$C-CH(CH3)2: a hexyne backbone (triple bond at C3, symmetric either direction) with two methyls on C2 and one methyl on C5 (numbering chosen to give the substituent locant SET {2,2,5}, lower than the alternative {2,5,5}): 2,2,5-trimethylhex-3-yne.

Step 4 (structure 4, from the common name). 'Ethyl isopropyl acetylene' means acetylene (HC$\equiv$CH) with an ethyl group on one triple-bond carbon and an isopropyl group on the other: Et-C$\equiv$C-iPr = CH3CH2-C$\equiv$C-CH(CH3)-CH3; numbering from the isopropyl end gives the methyl branch the lower locant (2 rather than 5): 2-methylhex-3-yne.

Step 5 (structure 5). CH$\equiv$C-C$\equiv$C-C$\equiv$CH is a six-carbon chain with three triple bonds, symmetric, starting at C1, C3 and C5: hexa-1,3,5-triyne.

For each structure: find the longest chain through all the multiple bonds, number it in the direction giving the lowest locant set (ene before yne when tied), name every substituent alphabetically with its locant, and assemble the full name.

• Numbering a chain from the 'wrong' end out of habit (e.g. always left-to-right) instead of checking BOTH directions and picking whichever gives the lower locant set for the multiple bonds first, then for substituents.
• Forgetting the IUPAC tie-break rule that, when the double-bond and triple-bond locant sets would be identical either way, DOUBLE bonds get the lower locant over triple bonds (relevant to structure 1).