E1 Flashcards
What is an E1 reaction?
Elimination reactions that take place by a stepwise mechanism involving intermediate carbocations, unimolecular
What is the rate of an E1 reaction?
k x [halogenoalkane]
Why is E1 unimolecular?
Only the halogenoalkane is involved in the transition state of the rate determining step therefore its unimolecular
What is a description of the E1 mechanism?
The reaction takes place in two steps, in the first step the C-X bond breaks to form a carbocation together with halide ion, this is the slower rate determining step which explains why the rate of the E1 reaction depends only on the concentration of the halogenoalkane, in the second step the base removes one of the hydrogen atoms on the beta carbon of the carbocation to form a new C=C bond
Why is there no stereoselectivity in the E1 mechanism?
The C-C bond in the intermediate carbocation can freely rotate to align the sigma orbital of the C-H bond with the empty p orbital. When these orbitals are parallel the orbitals overlap side on and the sp3 carbon atom next to the carbocation changes hybridisation to become an sp2 carbon atom and form the C=C pi bond
How do the number of alkyl groups bonded to the carbon atom in the C-X bond influence the rate of E1 reaction?
The greater the number of alkyl substituents the faster the E1 reaction, alkyl groups are electron donating +I groups and stabilise the intermediate carbocation. This affects the rate of the reaction because the more stable the carbocation is the faster it is formed by cleavage of the C-X bond in the first step of the reaction. This explains why tertiary halogenoalkanes readily undergo E1 reactions whereas E1 reactions of secondary halogenoalkanes are slower and primary halogenoalkanes do not undergo E1 reactions
What is the regioselectivity of the E1 reaction?
An E1 reaction of a halogenoalkanes can often form two structure isomers of an alkene, where this happens the major product is usually the most substituted structural isomer of the alkene, this is because the transition state leading to the more substituted alkene has the lower energy and so the more substituted alkene is formed faster
What is the stereoselectivity of an E1 reaction?
When an E1 reaction of a halogenoalkane can form two stereoisomers of an alkene the major stereoisomer is usually the alkene of E configuration. This is because the transition state leading to the E alkene has the lower energy and so is formed faster than the Z alkene. For example the conformation with the Ph and Et groups furthest apart is more stable because there is less steric strain
