4. Alkenes
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Are alkenes saturated or unsaturated?
Unsaturated
Do alkenes contain a double bond?
Yes
What is the arrangement of atoms around the carbon-carbon double bond in alkenes?
Trigonal planar
What is the bond angle of atoms around the carbon-carbon double bond in alkenes?
120°
What do the p orbitals on each C in the double bond overlap to form?
Two different types of bond; σ or π
What are the two ways that alkenes can react?
• by free radical addition * by electrophilic addition
How is a pi bond formed?
Due to the overlap of p-orbitals
What gives alkenes chemical properties that are not seen in alkanes?
The C-C double bond
What is an addition reaction?
The combination of two or more molecules to form a single molecule as a result of breaking the double bond
Which type of reaction is faster; addition or substitution?
Addition
Why are addition reactions generally faster than substitution?
Only weak π bonds are broken in addition, rather than strong σ bonds
What is an electrophile?
A species that can accept a pair of electrons from a species with high electron density
What is the area of high electron density in an alkene?
The σ-bond and π-bond
Why can σ and π bonds attract electrophiles?
They are areas of high electron density
In alkenes, after an electrophile is attracted to the σ or π bond, what happens?
The electrophile undergoes heterolytic fission
What is heterolytic fission?
Breaking of a covalent bond which results in both electrons going to the same atom
Why are alkenes much more reactive than alkanes?
They can undergo addition, and are able to react with electrophiles as well as free radicals
What is likely to happen in an alkene, considering that the bond is non-polar?
Homolytic fission may be undergone instead of heterolytic
What are symmetrical alkenes?
Where both carbon atoms in the double bond are attached to the same groups
What is it called when an atom has both carbon atoms in the double bond attached to the same groups?
Symmetrical
What are unsymmetrical alkenes?
Alkenes with the two carbons in the double bond not attached to the same groups
What are possible electrophiles in addition reactions with symmetrical alkenes?
• halogens (e.g. Br-Br) • hydrogen halides (e.g. H-Br) • H2SO4
What can symmetrical alkenes react with to undergo an addition reaction?
• halogens (e.g. Br-Br) • hydrogen halides (e.g. H-Br) • cold concentrated H2SO4
What are the stages in an addition reaction of a symmetrical alkene?
• delta +ve atom in electrophile attracted to electron rich C=C • double bond breaks and pair of electrons allow one of the C atoms to bond to the delta +ve atom • as a result second carbon in C=C is left electron deficient and a carbocation is formed • rest of electrophile molecule is electron rich with a lone pair and -ve charge - this is attracted to +ve C atom and lone pair used to form a bond (cancelling both charges)