Nucleophilic Substitiution Reactions
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What is the C-X bond and what does this mean?
it is polar and suseptible to attack by nucelophiles
What does the nucleophile attack?
δ+ carbon
Why is it called nucleophilic substitution?
the halgoen is replaced bu a nucleophile
Describe the Sn1 mechanism.
There are 2 steps. One species takes part in the rate determining step. In the first step the C-X bonds breaks heterolytically to form the halide ion and a carbocation intermediate. In the second step, a nucelophile rapidly attacks the positive carbon atom to form the product.
Why is the Sn1 said to be first order?
The overall rate of reaction depends on the slow step (Rate Determining Step) which involves only one molecule
What is the rate equation for the Sn1 mechanism?
Rate= k[haloalkane]
What is the carbocation intermediate stabilised by?
the alkyl groups, which are electron donating i.e. they push electrons onto the positive carbon atom thereby reducing its charge and making it more stable
What is the trend between alkyl groups and stabiliy of the carbocation?
The more alkyl groups there are, the more stable the carbocation
What class of haloalkane is more likely to undergo Sn1?
tertiary
Describe the Sn2 mechanism.
The nucleophile begins to form a bond between it and the carbom atom. At the same time, the bond between the carbon atom and the halogen starts to break. A transition state forms in which the new bond is half-formed and the existing bond is half-broken
What class of haloalkane is most likely to undergo Sn2 and why?
Primary In order for Sn2 to occur, the carbon atom it attacks cannot be too cluttered (Sterically Hindered)
What does the overall rate of reaction for Sn2 depend on?
Both reactants and, therefore, is second order
What is the rate equation for Sn2?
Rate= k[haloalkane][Nu]
What does nucleophilic substitution of haloalkanes by hydroxide ions produce?
alcohols
What does a base in water favour?
substitution
When are alkoxide ions formed?
when alcohols react with sodium metal
What can alkoxide ions be represented with?
R-O-
What are alkoxides said to be?
nucleophiles due to the negative charge
What do alkoxide ions react with haloalkanes to form?
ethers
What are ethers represented by?
R-O-R
What do haloalkanes react with cyanide ions to form?
nitriles
What does the reaction with cyanide ions do to the carbon chain?
increases the carbon chain length by one carbon
What are nitriles easily hydrolysed to?
a carboxylic acid