CHEM 2510: Chapter 3 Test Questions
and Answers.
What type of reaction do alkanes undergo? - Answer Radical reactions
What do radical reactions begin with? - Answer The breaking of a bond, also known as bond
dissociation
What is halogenation? - Answer A radical reaction in which a hydrogen atom in an alkane is
replaced by a halogen. An alkane becomes a haloalkane and a reactive functional group is
added
What is bond dissociation energy? - Answer The amount of energy needed to break a bond
and is a quantitative measure of bond strength. The higher the energy, the stronger the bond.
What is homolytic cleavage and what does it form? - Answer Homolytic cleavage is bond
breaking in such a way that the two bonding electrons divide equally between the two atoms.
This forms radicals.
What is heterolytic cleavage? - Answer Breaking a bond and the bonding electron pair is
donated to one of the atoms and forms ions.
Where is homolytic cleavage found? - Answer In nonpolar solvents, or the gas phase
Where is heterolytic cleavage found? - Answer Polar solvents
Do dissociation energies refer to both homolytic and heterolytic cleavage? - Answer No, they
only refer to homolytic cleavage. Bonds with high dissociation energies still undergo heterolytic
cleavage readily- DONT CONFUSE THE TWO!!!
When are bonds strongest? - Answer When made by overlapping orbitals that are close in size
and energy.
This is true in the case of bonding between C and the halogens.
What is the relationship between size of halogen and bond strength in haloalkanes? - Answer
As the halogen increases, its p orbital becomes larger and does not overlap as much, thus
Increasing size of halogen = Decreasing bond strength.
, What determines the C-H bond strenght? - Answer The stability of radicals.
What is the trend in strength from methane to tertiary carbon in C-H bonds? - Answer Bond
energies (= bond strength) generally decreases with the progression from methane to primary,
to secondary, to tertiary carbon
CH3H > Rprimary-H > Rsecondary -H > Rtertiary-H.
What is the trend in strength from methane to tertiary carbon in C-C bonds? - Answer A
similar trend to C-H bonds, with methane being the strongest and tertiary carbon being the
weakest.
What is the trend with radical stability? - Answer Stability increases along the series from
primary to secondary to tertiary. Consequently, the energy required to create the radicals
decreases.
"Tertiary has the weakest bond and most stable radical"
What configuration do methyl radicals adopt? - Answer A nearly planar configuration
What is hyperconjugation? - Answer When arrangement of a radical allows the bonding pair of
electrons in the sigma bond orbital to delocalize into the party empty p lobe. This interaction
has a stabilizing effect and is why the methyl radical is the least stable.
What does the geometry change form tetrahedral to planar do to increase stability of radicals? -
Answer It decreases steric crowding in the secondary and tertiary radicals
What is the difference between hyperconjugation and resonance? - Answer They are both
forms of electron delocalization, but are distinguished by the type of orbital. Resonance refers
to pi-type overlap of p orbitals, while hyperconjugation incorporates overlap with the sigma
bond orbitals.
When alkanes are heated to a high temp, what happens? - Answer Bond homolysis. Both C-H
and C-C bonds rupture, a process called pyrolysis.
What is radical combination? - Answer When, after cleavage into radicals, two radicals
combine to form a new chain.
What is hydrogen abstraction? - Answer When, after cleavage into radicals, a radical abstracts
a hydrogen from a chain, making the chain into a radical.
and Answers.
What type of reaction do alkanes undergo? - Answer Radical reactions
What do radical reactions begin with? - Answer The breaking of a bond, also known as bond
dissociation
What is halogenation? - Answer A radical reaction in which a hydrogen atom in an alkane is
replaced by a halogen. An alkane becomes a haloalkane and a reactive functional group is
added
What is bond dissociation energy? - Answer The amount of energy needed to break a bond
and is a quantitative measure of bond strength. The higher the energy, the stronger the bond.
What is homolytic cleavage and what does it form? - Answer Homolytic cleavage is bond
breaking in such a way that the two bonding electrons divide equally between the two atoms.
This forms radicals.
What is heterolytic cleavage? - Answer Breaking a bond and the bonding electron pair is
donated to one of the atoms and forms ions.
Where is homolytic cleavage found? - Answer In nonpolar solvents, or the gas phase
Where is heterolytic cleavage found? - Answer Polar solvents
Do dissociation energies refer to both homolytic and heterolytic cleavage? - Answer No, they
only refer to homolytic cleavage. Bonds with high dissociation energies still undergo heterolytic
cleavage readily- DONT CONFUSE THE TWO!!!
When are bonds strongest? - Answer When made by overlapping orbitals that are close in size
and energy.
This is true in the case of bonding between C and the halogens.
What is the relationship between size of halogen and bond strength in haloalkanes? - Answer
As the halogen increases, its p orbital becomes larger and does not overlap as much, thus
Increasing size of halogen = Decreasing bond strength.
, What determines the C-H bond strenght? - Answer The stability of radicals.
What is the trend in strength from methane to tertiary carbon in C-H bonds? - Answer Bond
energies (= bond strength) generally decreases with the progression from methane to primary,
to secondary, to tertiary carbon
CH3H > Rprimary-H > Rsecondary -H > Rtertiary-H.
What is the trend in strength from methane to tertiary carbon in C-C bonds? - Answer A
similar trend to C-H bonds, with methane being the strongest and tertiary carbon being the
weakest.
What is the trend with radical stability? - Answer Stability increases along the series from
primary to secondary to tertiary. Consequently, the energy required to create the radicals
decreases.
"Tertiary has the weakest bond and most stable radical"
What configuration do methyl radicals adopt? - Answer A nearly planar configuration
What is hyperconjugation? - Answer When arrangement of a radical allows the bonding pair of
electrons in the sigma bond orbital to delocalize into the party empty p lobe. This interaction
has a stabilizing effect and is why the methyl radical is the least stable.
What does the geometry change form tetrahedral to planar do to increase stability of radicals? -
Answer It decreases steric crowding in the secondary and tertiary radicals
What is the difference between hyperconjugation and resonance? - Answer They are both
forms of electron delocalization, but are distinguished by the type of orbital. Resonance refers
to pi-type overlap of p orbitals, while hyperconjugation incorporates overlap with the sigma
bond orbitals.
When alkanes are heated to a high temp, what happens? - Answer Bond homolysis. Both C-H
and C-C bonds rupture, a process called pyrolysis.
What is radical combination? - Answer When, after cleavage into radicals, two radicals
combine to form a new chain.
What is hydrogen abstraction? - Answer When, after cleavage into radicals, a radical abstracts
a hydrogen from a chain, making the chain into a radical.
