Rank fastest to slowest reaction with HBr:
A) H3C-C---C-CH3 (alkyne; that's a triple bond between the carbons)
B) H CH3 (alkene; trans)
=
H3C H
C) H3C CH3 (alkene; 3 methyl substituents)
=
H3C H
Please explain why. Thanks!
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To determine the relative reaction rates of the given compounds with HBr, we need to consider the stability of the resulting carbocation intermediates formed during the reaction. The more stable the intermediate, the faster the reaction will occur.
In this case, let's assess the stability of the carbocation intermediates formed during the reaction with HBr for each compound:
A) H3C-C---C-CH3 (alkyne; triple bond between carbons):
When HBr reacts with the alkyne, a carbocation intermediate is formed. The resulting carbocation will be connected to two carbon atoms bonded by triple bonds. Since triple bonds are more electronegative than double bonds or single bonds, they can stabilize the positive charge on the carbocation more effectively. Thus, the intermediate formed in this case will be relatively stable, resulting in a relatively fast reaction rate.
B) H CH3 (alkene; trans)
=
H3C H
In this alkene, a carbocation intermediate is formed when HBr reacts. However, the resulting carbocation will have only one carbon atom bonded by a double bond. Double bonds are less electronegative than triple bonds and, therefore, provide less stabilizing effect to the carbocation intermediate. Consequently, the reaction rate will be slower compared to the alkyne.
C) H3C CH3 (alkene; three methyl substituents)
=
H3C H
In this alkene with three methyl substituents, a carbocation intermediate is formed during the reaction with HBr. The resulting carbocation will also have only one carbon atom bonded by a double bond, similar to compound B. Since there are no additional electron-donating groups present to stabilize the carbocation, the reaction rate will be slower than that of compound A (alkyne) but faster than that of compound B (alkene with trans configuration).
Therefore, the ranking from fastest to slowest reaction rate with HBr is:
1) A) H3C-C---C-CH3 (alkyne; triple bond between carbons)
2) C) H3C CH3 (alkene; three methyl substituents)
3) B) H CH3 (alkene; trans)