A variety of phenyl-substituted acetylenes (1a–d) were treatedwith HCl to give a mixture of E and Z isomers, as shown below:
A Variety Of Phenyl Substituted Acetylenes 1a D Were Treated With Hcl To Give A Mixture Of E And Z Isomers As Shown B 1 (48.51 KiB) Viewed 46 times
a) VDraw the vinyl carbocation that is predominantly formed whenphenyl-substituted acetylenes are protonated by HCl and explain theregioselectivity observed in these reactions.
b) By comparing transition state stabilities for the step inwhich the vinyl carbocation is captured by a chloride ion, providean explanation for the stereoselectivity (the E:Z ratio) observedin the reactions of 1a–d.
1 HCI -CEC-R R C || H 1a (R = Me) 1b (R = Et) 1c (R = i-Pr) 1d (R = t-Bu) E:Z Ratio 70:30 80:20 95:5 100:0 C CI R
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