Loudon Ch. 4 Review: Alkene Structure & Reactivity
Jacquie Richardson, CU Boulder – Last updated 9/2/2020
In this chapter we’re looking at alkenes – compounds with carbon-carbon double bonds.
Alkene Nomenclature
All the rules from alkane naming still apply, but they’re all overridden by a few new rules that
apply when double bonds show up.
1. Determine the longest/principal/parent chain. This is the chain with the most
double bonds in it, even if this is shorter than a longer chain with fewer alkenes. Here is
an example of the parent chain not being the longest chain anymore. If there are
multiple chains that are tied for most double bonds, then choose the longest chain from
among them.
2. Name each group coming off the parent chain. Same as before, but there are a
couple of new alkene-containing substituents that we’ll see here: vinyl and allyl. These
get alphabetized and listed the same as any other alkyl branch group.
3. Number the carbons of the parent chain. Start numbering at the end closest to the
first double bond, or, if both ends are equally close to the first double bond, choose the
end that gives the lower number at the first point of difference for all double bonds. If
the locations of the double bonds are the same no matter which end you count from,
then you can start using the alkane rules to determine it.
4. Name the compound as “location #”-“substituent”“parent”. Drop “ane” from the
end of the parent name and add “ene” if there’s one double bond, “adiene” if there’s
two, “atriene” if there’s three, etc. To describe the location of double bonds, put the
numbers of the first carbon of each double bond either before the parent chain name, or
in the middle of the parent chain name before the new ending.
Describe the location of substituents the same as before.
5. Sometimes the parent is a ring, instead of a chain. For naming rings, you still want
to minimize the location of the double bond, so all cycloalkenes will have a double bond
between carbons 1 and 2 of the ring (so if there’s only one double bond, this location
identifier can be dropped.) But that still leaves ambiguities about naming which can be
resolved by the old alkane rules about minimizing branch locations.
1
, Loudon Ch. 4 Review: Alkene Structure & Reactivity
Jacquie Richardson, CU Boulder – Last updated 9/2/2020
6. In some cases, we will see substituents-of-substituents. This is similar to alkyl
substituents – the point of attachment to the parent is always carbon #1 of the group,
but the ending changes to “enyl” rather than “yl”, and you will need to include
information about the location of the double bond within the group. Once there are any
numbers inside the substituent name, you have to put the whole thing in parentheses.
Alkenes & Orbital Geometry
We already saw in Ch. 1 that π bonds are based on side-on overlap of leftover p orbitals.
There are a couple of consequences of this side-on overlap that forms the π bond. For one
thing, p orbitals have to be in the same plane to overlap, so the C=C double bond is not
freely rotating the way a σ bond would be. For another, π bonds are more easily broken than
σ bonds since side-on overlap is not as strong as head-on overlap. This is why we showed π
bonds breaking during the formation of new resonance states in Ch. 3.
Alkene Stereoisomers
Not only are constitutional isomers possible, based on the location of the double bond:
…but also the double bond itself is locked into one of two possible positions, because it’s
not freely rotating. When each double-bonded carbon has one H and one non-H, these two
options are called cis and trans. This descriptor goes in front of the overall compound name,
in italics.
The two versions of this molecule have identical connectivity but different spatial
arrangement, which makes them stereoisomers (specifically, cis-trans stereoisomers). If you
swap the two groups connected to one of the double-bonded Cs, you’ll interconvert
2
Jacquie Richardson, CU Boulder – Last updated 9/2/2020
In this chapter we’re looking at alkenes – compounds with carbon-carbon double bonds.
Alkene Nomenclature
All the rules from alkane naming still apply, but they’re all overridden by a few new rules that
apply when double bonds show up.
1. Determine the longest/principal/parent chain. This is the chain with the most
double bonds in it, even if this is shorter than a longer chain with fewer alkenes. Here is
an example of the parent chain not being the longest chain anymore. If there are
multiple chains that are tied for most double bonds, then choose the longest chain from
among them.
2. Name each group coming off the parent chain. Same as before, but there are a
couple of new alkene-containing substituents that we’ll see here: vinyl and allyl. These
get alphabetized and listed the same as any other alkyl branch group.
3. Number the carbons of the parent chain. Start numbering at the end closest to the
first double bond, or, if both ends are equally close to the first double bond, choose the
end that gives the lower number at the first point of difference for all double bonds. If
the locations of the double bonds are the same no matter which end you count from,
then you can start using the alkane rules to determine it.
4. Name the compound as “location #”-“substituent”“parent”. Drop “ane” from the
end of the parent name and add “ene” if there’s one double bond, “adiene” if there’s
two, “atriene” if there’s three, etc. To describe the location of double bonds, put the
numbers of the first carbon of each double bond either before the parent chain name, or
in the middle of the parent chain name before the new ending.
Describe the location of substituents the same as before.
5. Sometimes the parent is a ring, instead of a chain. For naming rings, you still want
to minimize the location of the double bond, so all cycloalkenes will have a double bond
between carbons 1 and 2 of the ring (so if there’s only one double bond, this location
identifier can be dropped.) But that still leaves ambiguities about naming which can be
resolved by the old alkane rules about minimizing branch locations.
1
, Loudon Ch. 4 Review: Alkene Structure & Reactivity
Jacquie Richardson, CU Boulder – Last updated 9/2/2020
6. In some cases, we will see substituents-of-substituents. This is similar to alkyl
substituents – the point of attachment to the parent is always carbon #1 of the group,
but the ending changes to “enyl” rather than “yl”, and you will need to include
information about the location of the double bond within the group. Once there are any
numbers inside the substituent name, you have to put the whole thing in parentheses.
Alkenes & Orbital Geometry
We already saw in Ch. 1 that π bonds are based on side-on overlap of leftover p orbitals.
There are a couple of consequences of this side-on overlap that forms the π bond. For one
thing, p orbitals have to be in the same plane to overlap, so the C=C double bond is not
freely rotating the way a σ bond would be. For another, π bonds are more easily broken than
σ bonds since side-on overlap is not as strong as head-on overlap. This is why we showed π
bonds breaking during the formation of new resonance states in Ch. 3.
Alkene Stereoisomers
Not only are constitutional isomers possible, based on the location of the double bond:
…but also the double bond itself is locked into one of two possible positions, because it’s
not freely rotating. When each double-bonded carbon has one H and one non-H, these two
options are called cis and trans. This descriptor goes in front of the overall compound name,
in italics.
The two versions of this molecule have identical connectivity but different spatial
arrangement, which makes them stereoisomers (specifically, cis-trans stereoisomers). If you
swap the two groups connected to one of the double-bonded Cs, you’ll interconvert
2
