13.4 Electrophillic addition of alkenes Flashcards
(24 cards)
Definition of electrophile
An electron pair acceptor
What reactions do alkenes take part in?
Addition reactions to form saturated compounds
What is the name of the mechanism for the addition reactions of alkenes to form saturated compounds?
Electrophilic addition
Which region of an alkene attracts an electrophile and why?
-The double bond, it is a region of high electron density die to the presence of π electrons, this region of high electron density attracts the electrophiles
What is the mechanism for the reaction between but-2-ene and hydrogen bromide?
Explain the mechanism for the reaction between but-2-ene and hydrogen bromide?
- Br is more electronegative than H so HBr is a polar molecule and contains the dipole Hδ+—Br δ-
- The electron pair in the π bond is attracted to the δ+ on the H causing the double bond to break
- A bond forms between the hydrogen atoms from the HBr and a carbon atom that was apart of the double bond
- The HBr bond breaks by heterolytic fission, with the electron pair going to the bromine atom
- An bromine ion (Br-) and a carbocation are formed
- The Br- ions react with the carbocation to from the addition product
Definition of a carbocation
An ion that contains a positively changed carbon atom
What type of bond breaking occurs in electrophillic addition?
Heterolytic
What is the mechanism for electrophilic addition of propene and bromine?
What is Markownikoff’s rule?
When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the halide will attach itself to the C with the most number of hydrogens and smaller number of carbons
What are the two types of carbocations formed in the first step of electrophillic addition?
Primary, secondary
When is the positive change for a primary and secondary carbocation?
Primary - on the carbon atom at the end of the carbon chain
Secondary - on a carbon atom with two carbon chains attatched
Which carbocation is the major and minor product formed by?
Major product - secondary carbocation
Minor product - primary carbocation
What are the two possible carbocations formed when propene reacts with a hydrogen halide (HBr)?
How are carbocations classified?
By the number of alkyl groups attached to the positively changed carbon atom
How are alkyl groups represented?
-R
Which type of carbocation is the most stable?
Tertiary carbocations
What are the least stable carbocations?
Primary
What is a primary carbocation?
One carbon chain (alkyl group) attached to the positive carbon
What is a secondary carbocation?
Two carbon chains (alkyl groups) attached to the positive carbon
What is a tertiary carbocation?
Three carbon chains (alkyl groups) attached to the positive carbon
How is carbocation stability explained?
Each alkyl group donates and pushes electrons towards the positive charge of the carbocation. The more alkyl groups attached to the positively charged carbon atom, the more the charge is spread out, making the ions more stable
What is the major product?
Formed from the most stable carbocation
