Nucleophilic mechanism - alcohols alkyl halides and amines Flashcards
What are the two types of amines
Aliphatic (attached to hydrocarbon chain) or aromatic (attached to benzene ring)
Are alcohols water soluble?
Yes, only small alcohols up to 5C
Are amines water soluble?
Yes, only small amines up to 5C
Are alkyl halides water soluble?
No, they are non-polar despite containing an electronegative halogen. This makes them able to dissolve non-polar substances and hence they are great organic solvents.
What is nucleophilic substition?
Replacement of one nucleophile with another
Are halogens of an alkyl halide good or bad leaving groups?
They are excellent leaving groups as they are considered weak bases
Rank the halogens in order of ability to leave a molecule?
I > Br > Cl
What are examples of poor leaving groups?
H, NH2, OH, RO
What are the two mechanisms for nucleophilic substitution?
SN1 (unimolecular) and SN2 (bimolecular) mechanisms
What is the SN1 mechanism and when does it apply?
SN1 mechanism is the unimolecular mechanism - hence its rds depends on a single molecule (the electrophile)
Thus, SN1 acts as a two step process involving a carbocation intermediate therefore molecules which form a more stable carbocation favour SN1 (tertiary>secondary>primary)
The quality of the leaving group determines the rate at which the reaction occurs (good leaving groups will mean faster rate of reaction)
What happens to the stereochemistry of R and S enantiomers undergoing nucleophilic substitution by SN1 mechanism?
The carbocation that forms is sp2 hybridised forming a planar shape. Hence, the nucleophile is able to join from the top or the bottom therefore it will lead to the formation of a racemic mixture with an even composition of both R and S enantiomers.
What is the SN2 mechanism and when does it apply?
SN2 mechanism is the bimolecular mechanism - hence its rds depends on two molecules - the electrophile and the nucleophile
Thus, SN2 is a one step process with a transition state in which both the nucleophile and the leaving group are bonded to the carbon atom at the same time. Because there is no involvement of a carbocation intermediate molecules favour SN2 in order of primary>secondary>tertiary. Also, tertiary electrophiles will cause more steric interference as the transition state forms
What happens to the stereochemistry of R and S enantiomers undergoing nucleophilic substitution by SN2 mechanism?
As the nucleophile comes in at one side the transition stage will have the nucleophile and leaving group at opposite ends of the molecule. As the leaving group breaks its bond to the carbon atom we get an inversion of configuration meaning all of R and S enantiomers are exchanged.
What determines the strength of a nucleophile?
Nucleophiles are electron donors = hence, the availability of a pair of electrons based on their electronegativity affects the strength of the nucleophile?
What are examples of weak nucleophiles?
Compounds with neutral oxygens will be weak nucleophiles as oxygen has a high electronegativity meaning its lone pair less available for reaction
e.g. water, alcohols, carboxylic acids
Weak nucleophiles are more likely to act via the SN1 mechanism as the quality of the nucleophile does not affect the rate of reaction in this case.