Alkenes Flashcards
What is an alkene also known as?
Olefin
what is an alkene?
- An unsaturated hydrocarbon compound
- Containing at least one carbon-carbon double bond
What is the general formula for alkenes?
CnH2n for straight chains.
2 hydrogen atoms are subtracted for each ring
What is the functional group in alkenes? Why?
The double bond - it determines the chemical properties of alkenes
How are double bonds hybridised?
sp2
How does the nomenclature for alkenes differ from that for alkanes?
It’s the same except:
- “Ene” replaces “ane”
- The double bond is(are) numbered, trying to get the smallest possible number
Why do trans compounds tend to have higher melting points and lower boiling points?
Better symmetry and less polarity, respectively
Can alkenes be polar?
Yes, because of the double bond
What affect does the number of alkyl groups have on an alkene’s stability?
Greater number of alkyl groups (i.e. the more substituted the double bond), the greater the alkene’s stability.
The reason is that alkyl groups are somewhat electron donating, thus they stabilise the double bond
What is an alkene with two double bonds called? What about if it has a single bond between the two double bonds?
A diene.
A conjugate diene.
Conjugated dienes are more stable than non-conjugated ones dienes primarily due to the resonance stabilisation
What is an alkene with three double bonds called?
A triene
What are electrophiles?
What happens when they add to alkenes?
Substances which seek electrons.
Carbocations (carbonium ions) are formed
Name an important electrophile
H+
What is a nucleophile?
A molecule with a free pair of electrons, and sometimes a negative charge, that seeks out partially or completely positively charged species.
I.e. a carbon nucleus. Some important nucleophiles are OH- and CN-
True or false, the double bond has the ability to to stabilise carbocations, carboanions and radicals attached to adjacent carbons (allylic carbons)
True
What does the stability of an intermediate carbon depends on?
In general what effect do groups which can share electrons by pi orbital overlap (resonance) have on a carbocation?
The groups attached to it. They can either stabilise or destabilise it.
They stabilise it.
What effect do groups which place partial or total positive charge adjacent to a carbocation and thus withdraw electrons inductively by sigma bonds, have on a carbocation?
They destabilise it
The intermediate carbocation must be what?
The most stable.
This is the reason for Markovnikoff’s rule
What is Markovnikoff’s rule?
It states that the nucleophile will be bonded to the most substituted carbon (fewest hydrogens attached) in the product. Equivalently the electrophile will be bonded to the least substituted carbon (most hydrogens attached) in the product
What would happen if an H+ added to the most substituted carbon?
The anti-Markovnikoff product would be formed (this is much less likely)
Under what conditions does the opposite of the Markovnikoff rule apply?
Radical conditions.
Thus anti-Markovnikoff products are the major products under free radical conditions
How does ozone (O3) react with alkenes?
Vigorously.
The reaction ozonolysis leads to an oxidative cleavage of the double bond which can produce a ketone and an aldehyde
What reaction is it when one atom of hydrogen adds to each carbon of a double bond (hydrogenation)?
An addition reaction
Why is the process of hydrogenation referred to as a heterogenous catalysis?
There are two phases present (the hydrogen and the metal catalyst)
When may hydrogens react with alkenes?
In the presence of a variety of metal catalysts (such as nickel, palladium, platinum)
A carbon with multiple bonds is not bonded to the maximum number of atoms that potentially a carbon could possess. What can thus be said about it?
It is unsaturated
Why does one say that alkanes are saturated?
Since each carbon is bonded to the maximum number of atoms that it could possess (four).
Hydrogenation is sometimes called the process of saturation
What is an alkyne?
An unsaturated hydrocarbon molecule containing carbon-carbon triple bonds.
Nomenclature the same as that for alkenes, except that “yne” is used as the suffix.
Alkynes have similar physical properties and chemical reactions (i.e. electrophilic addition, oxidation) to alkenes.
Current GAMSAT exam does not require any knowledge specific to alkyne chemistry
