ORGANIC CHEMISTRY EXAM 2
QUESTIONS AND ANSWERS GRADED A+
2025/2026
Which is more stable: trans- or cis- isomer? - ANS Trans-
(In cis- molecule, larger substituents are forced to occupy the same plane, thereby decreasing
stability)
Which is more stable: alkene with one alkyl substituent on the double bond or two? -
ANS Two
(Alkenes are stabilized by alkyl substituents on the double bond. The alkene with the greatest
number of substituents is most stable. Identity of substituents not as important as number of
them-- i.e. alkene with two smaller alkyl substituents still more stable than alkene with one
large alkyl substituent)
Products: Addition of Hydrogen Halides to Alkenes - ANS Produces alkyl halides, in which
halogen is bonded to saturated carbon atom
Main product is isomer in which the halogen is bonded to the carbon of the double bond with
the greater number of alkyl substituents, and the hydrogen is bonded to the carbon with the
smaller number of alkyl substituents
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,Size of substituents does not matter
Mechanism: Addition of Hydrogen Halides to Alkenes - ANS 1) Electron pair in the pi bond of
an alkene is donated to the hydrogen of the hydrogen halide. As a result, the carbon-carbon
double bond is protonated on the carbon atom with smaller number of alkyl substituents. The
other carbon (the one with the greater number of alkyl substituents) becomes positively
charged and electron-deficient (called a carbocation). Lewis acid.
2) Halide ion, which is a Lewis base, or nucleophile, reacts with the carbocation at it electron-
deifient carbon atom.
Stability of Carbocations - ANS Carbocations are classified by the degree of alkyl substitution
at their electron-deficient carbon atoms. Alkyl substituents at the electron-deficient carbon
strongly stabilize carbocations. Therefore,
tertiary>secondary>primary
Hyperconjugation - ANS Overlap of bonding electrons from the adjacent sigma bonds with
the unoccupied 2p orbital of the carbocation. Causes stability of carbocations with greater
number of alkyl substituents
(Energetic advantage of hyperconjugation is that it involves additional bonding. That is, the
electrons in the C-H bonds participate in bonding not only with the C and H, but also with the
electron-deficient carbon)
Carbocation Rearrangement in Hydrogen Halide Addition - ANS In a rearrangement, a group
from the starting material has moved to a different position in the product. (Group must come
from a carbon directly attached to the electron-deficient, positively charged carbon of the
carbocation)
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, Can take the form of an alkyl group moving (with its bonding pair of electrons) from one carbon
to another. Could also be a hydride shift (migration of a hydrogen with its two bonding
electrons)
Essentially a Lewis acid-base reaction in which the electron-deficient carbon is the Lewis acid
and the migrating group is the Lewis base
In both cases, one carbocation is converted into a different, more stable carbocation (almost
always occurs when possible).
If choice between alkyl or hydride group moving, hydride migration typically occurs because it
gives the more stable carbocation
Transition State - ANS The rate of a chemical reaction can be defined for our purposes as the
number of reactant molecules converted into product in a given time. The theory of reaction
rates used by many organic chemists postulates that as the reactants change into products, they
pass through an unstable state of maximum free energy, called the transition state. The
transition state has a higher energy than either the reactants or products and therefore
represents an energy barrier (called the standard free energy of activation) to their
interconversion. The higher the barrier, the smaller the rate.
A reaction and its reverse have the same transition state and, therefore, the same energy of
activation.
Each step of a multistep reaction has its own characteristic rate and therefore its own transition
state. It often happens that one step of a multistep reaction is considerably slower than any of
the others. This slowest step is called the rate-limiting step and it is the one with the transition
state of highest free energy. In such cases, the rate of the overall reaction is equal to the rate of
the rate-limiting step
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QUESTIONS AND ANSWERS GRADED A+
2025/2026
Which is more stable: trans- or cis- isomer? - ANS Trans-
(In cis- molecule, larger substituents are forced to occupy the same plane, thereby decreasing
stability)
Which is more stable: alkene with one alkyl substituent on the double bond or two? -
ANS Two
(Alkenes are stabilized by alkyl substituents on the double bond. The alkene with the greatest
number of substituents is most stable. Identity of substituents not as important as number of
them-- i.e. alkene with two smaller alkyl substituents still more stable than alkene with one
large alkyl substituent)
Products: Addition of Hydrogen Halides to Alkenes - ANS Produces alkyl halides, in which
halogen is bonded to saturated carbon atom
Main product is isomer in which the halogen is bonded to the carbon of the double bond with
the greater number of alkyl substituents, and the hydrogen is bonded to the carbon with the
smaller number of alkyl substituents
1 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED.
,Size of substituents does not matter
Mechanism: Addition of Hydrogen Halides to Alkenes - ANS 1) Electron pair in the pi bond of
an alkene is donated to the hydrogen of the hydrogen halide. As a result, the carbon-carbon
double bond is protonated on the carbon atom with smaller number of alkyl substituents. The
other carbon (the one with the greater number of alkyl substituents) becomes positively
charged and electron-deficient (called a carbocation). Lewis acid.
2) Halide ion, which is a Lewis base, or nucleophile, reacts with the carbocation at it electron-
deifient carbon atom.
Stability of Carbocations - ANS Carbocations are classified by the degree of alkyl substitution
at their electron-deficient carbon atoms. Alkyl substituents at the electron-deficient carbon
strongly stabilize carbocations. Therefore,
tertiary>secondary>primary
Hyperconjugation - ANS Overlap of bonding electrons from the adjacent sigma bonds with
the unoccupied 2p orbital of the carbocation. Causes stability of carbocations with greater
number of alkyl substituents
(Energetic advantage of hyperconjugation is that it involves additional bonding. That is, the
electrons in the C-H bonds participate in bonding not only with the C and H, but also with the
electron-deficient carbon)
Carbocation Rearrangement in Hydrogen Halide Addition - ANS In a rearrangement, a group
from the starting material has moved to a different position in the product. (Group must come
from a carbon directly attached to the electron-deficient, positively charged carbon of the
carbocation)
2 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED.
, Can take the form of an alkyl group moving (with its bonding pair of electrons) from one carbon
to another. Could also be a hydride shift (migration of a hydrogen with its two bonding
electrons)
Essentially a Lewis acid-base reaction in which the electron-deficient carbon is the Lewis acid
and the migrating group is the Lewis base
In both cases, one carbocation is converted into a different, more stable carbocation (almost
always occurs when possible).
If choice between alkyl or hydride group moving, hydride migration typically occurs because it
gives the more stable carbocation
Transition State - ANS The rate of a chemical reaction can be defined for our purposes as the
number of reactant molecules converted into product in a given time. The theory of reaction
rates used by many organic chemists postulates that as the reactants change into products, they
pass through an unstable state of maximum free energy, called the transition state. The
transition state has a higher energy than either the reactants or products and therefore
represents an energy barrier (called the standard free energy of activation) to their
interconversion. The higher the barrier, the smaller the rate.
A reaction and its reverse have the same transition state and, therefore, the same energy of
activation.
Each step of a multistep reaction has its own characteristic rate and therefore its own transition
state. It often happens that one step of a multistep reaction is considerably slower than any of
the others. This slowest step is called the rate-limiting step and it is the one with the transition
state of highest free energy. In such cases, the rate of the overall reaction is equal to the rate of
the rate-limiting step
3 @COPYRIGHT 2025/2026 ALLRIGHTS RESERVED.
