(i) An alkene 'A' on ozonolysis gives a mixture of ethanal and pentan-3-one. Write the structure and IUPAC name of 'A'. (ii) Why is Wurtz reaction not preferred for the preparation of alkanes containing odd number of carbon atoms? **OR** Write chemical equations for the combust

Question

(i) An alkene 'A' on ozonolysis gives a mixture of ethanal and pentan-3-one. Write the structure and IUPAC name of 'A'. (ii) Why is Wurtz reaction not preferred for the preparation of alkanes containing odd number of carbon atoms?  **OR**  Write chemical equations for the combustion reactions of the following hydrocarbons: (A) Butane (B) Pentene (C) Hexyne (D) Toluene.

Accepted Answer

>

> [!TLDR]
> (i) Reassembling the ozonolysis fragments of ethanal and pentan-3-one gives the alkene 3-ethylpent-2-ene. (ii) Wurtz reaction couples two identical alkyl halides, which structurally always produces an even-carbon alkane; making an odd-carbon alkane needs two different halides, which gives an unavoidable statistical mixture of three products.
>
> (i) Structure and IUPAC name of alkene A:
> Ozonolysis of an alkene cleaves the C=C double bond and replaces each carbon of the double bond with a C=O (carbonyl) group, giving two carbonyl compounds. To find the original alkene, we reverse this: join the two carbonyl carbons back together with a double bond, removing the two oxygens.
>
> - Ethanal is CH3-CHO. Removing the oxygen from its carbonyl carbon leaves the fragment CH3-CH= .
> - Pentan-3-one is CH3-CH2-CO-CH2-CH3 (a symmetrical ketone with the carbonyl at C3, bonded to two ethyl groups). Removing the oxygen from its carbonyl carbon leaves the fragment =C(CH2CH3)2 .
>
> Joining these two fragments with a double bond gives:
> CH3-CH=C(CH2CH3)2
>
> Numbering the longest chain containing the double bond (5 carbons, taking one of the ethyl groups as part of the main chain): C1(CH3)-C2(=CH)-C3(=C, bearing an ethyl substituent)-C4(CH2)-C5(CH3). This is pentene with the double bond starting at C2 and an ethyl substituent at C3.
>
> IUPAC name: 3-ethylpent-2-ene.
>
> Check: ozonolysis of 3-ethylpent-2-ene cleaves the C2=C3 bond. The C2 side (CH3-CH=) becomes CH3-CHO (ethanal); the C3 side (=C bonded to an ethyl substituent and to a -CH2CH3 continuation of the main chain, i.e. bonded to two ethyl groups overall) becomes (C2H5)2C=O, which is pentan-3-one. This matches the given products, confirming the structure.
>
> (ii) Why Wurtz reaction is not preferred for odd-carbon alkanes:
> The Wurtz reaction is: 2 R-X + 2Na -> R-R + 2NaX (dry ether, via a free-radical coupling mechanism). When a single alkyl halide R-X is used, the product R-R always has double the num

Q.(i) An alkene 'A' on ozonolysis gives a mixture of ethanal and pentan-3-one. Write the structure and IUPAC name of 'A'.

Concept understanding — Ozonolysis of Alkenes

Ozonolysis of Alkenes – From Intuition to Precision

The Precise Statement

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Q.(i) An alkene 'A' on ozonolysis gives a mixture of ethanal and pentan-3-one. Write the structure and IUPAC name of 'A'.

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