7. Nucleophilic Substitution and Elimination Reactions Flashcards
what are nucleophiles?
a molecule that tends to give away its electrons to electrophiles
what are electrophiles?
molecules that tend to accept electrons
how are nucleophiles and leaving groups related in terms of basicity?
they are conjugate bases
what are leaving groups?
parts of a molecule that detach during substitution and elimination reactions
what is the leaving group like if the nucleophile is good?
the leaving groups is bad because it is unstable and reactive
what is the leaving group like if the nucleophile is bad?
the leaving groups is good because it is stable and unreactive
what must nucleophiles have in terms of electrons and why?
a lone pair to attack electrophiles
when does nucleophilicity increase in terms of charge?
the more negative charge
why are electrophiles subject to attack by nucleophiles?
because they are electron deficient and nucleophiles are attracted to molecules that are more positively charged
what are the most common electrophiles?
- a carbon at the base of a carbonyl
- carbocation
when does nucleophilicity increase in terms of basicity?
the more basic
when does nucleophilicity increase in terms of size?
the bigger an atom is
what are the different types of electrophiles?
- carbocations
- Lewis acids
- molecules with a partial positive charge
what properties must leaving groups have?
- electronegative halogen
- resonance
- induction
what are substitution reactions?
reactions that replace the leaving group with a nucleophile
what are elimination reactions?
reactions where once a base is added to a molecule to take away the protons of a molecule, a leaving group leaves and leaves behind a double bond
what is SN1 reaction?
a unimolecular reaction meaning only one species is present in the reaction
what does for instance increasing the concentration of a species in a SN1 reaction do to the rate of the reaction?
increases the rate of the reaction
What does SN1 in the SN1 reaction stand for?
Substitution, Nucleophilic, Unimolecular
what is the rate equation in SN1 reactions?
rate=k[substrate]
what are weak nucleophiles in terms of amount of electrons?
atoms with electrons close to having a full octet
what molecules are included in an SN1 reaction?
a weak nucleophile and an electrophile with a lot of steric hindrance
what happens if you produce a chiral carbon in an SN1 reaction?
you get a race mixture, an equal mixture of two different enantiomers
why do SN1 reactions include a weak nucleophile and an electrophile with a lot of steric hindrance?
Electrophiles with a lot of steric hindrance usually have bulky groups making it difficult for the nucleophile to attack. This type of reaction favours slower nucleophilic attacks because the nucleophile is weak
how do you rank the reactivity of molecules in SN1 reactions?
- rewrite the molecules with the leaving group gone
- add a positive charge to the carbon the leaving group left from
- rank the molecules, the more substituted the carbocation, the more reactive the molecule is
what is an SN2 reaction?
a bimolecular reaction where two species are present in the reaction
what is the reaction rate equation for SN2 reactions?
rate=k[substrate][nucleophile]
what does increasing the concentration of a species in an SN2 reaction do to the rate of the reaction?
increases the reaction rate
what molecules are included in SN2 reactions?
a strong nucleophile and an electrophile with little steric hindrance
how are sodium and potassium related in terms of nucleophiles?
they are both strong nucleophiles
what are the steps in an SN2 reaction?
- start off with a nucleophile and a chiral carbon molecule
- a nucleophile replaces hydrogen in a chiral carbon molecule as the intermediate step
- the halogen leaves as the final step
what are the products in SN2 reactions like in terms of stereochemistry?
a change in position of the original stereochemistry
when is stereochemistry considered in substitution reactions?
when a chiral product is formed
what needs to happen to alcohol as a leaving group and why?
alcohol needs to be converted into a different functional group because it is a terrible leaving group
what are the products in SN1 reactions like in terms of stereochemistry?
race mixture, two enantiomers
why are alcohols a bad leaving group? (3)
- alcohols are strong bases so they are less likely to stabilize after they leave
- the oxygen in the alcohol may cause it to be too large to leave
- alcohols are stabilizing solvents so they may be less likely to leave the overall molecule because it is stabilizing it
what are four ways to convert alcohol into a better leaving group?
- convert to water through protonation
- convert into a tosyl through tosylation
- convert to a methyl through methylation
- convert to an alkyl halide through
how does an alcohol group turn into an alkyl halide?
instead of the side chain attaching to the alcohol it attaches to a halide
describe protonation to make alcohol a better-leaving group.
a hydrogen is added to the alcohol to turn it into water
what type of solvent is used in SN2 reactions and why?
polar aprotic solvent because fewer protons allow the strong nucleophile to swap places with the leaving group more easily
what type of solvent is used in SN1 reactions and why?
polar protic solvent because more protons allow the leaving group to leave more easily
what is an E1 reaction?
An unimolecular reaction where one species is present in the reaction
what does increasing the concentration of a species in an E1 reaction do to the rate of the reaction?
increases the rate of the reaction
what molecules are included in E1 reactions?
a weak base and an electrophile with a lot of steric hindrance
what does E1 in the E1 reaction stand for?
Elimination, Unimolecular
what is the rate equation of E1 reactions?
rate=k[substrate]
what are the steps of an E1 reaction?
- start off with a molecule that has a halide
- the halide is removed leaves behind a positive charge of carbon
- an extra bond goes to where the carbocation is and the positive charge disappears
what is an E2 reaction?
a bimolecular reaction where two species are present
what does increasing the concentration of a species in an E2 reaction do to the rate of the reaction?
increase the rate of the reaction
what is the reaction rate equation of an E2 reaction?
rate=k[substrate][base]
what molecules are included in an E2 reaction?
a strong base and an electrophile with little steric hindrance
what is the planar relationship between the hydrogen being deprotonated and the leaving group?
anitiperiplanar
what is a vicinal dihalide?
a molecule with two halogens on opposite sides of a bond
what are the steps of an E2 reaction?
- start off with a molecule that has a halide
- the oxygen from the hydroxyl group removes hydrogen from a carbon
- the carbon with the removed hydrogen turns into a double bond
- the leaving group comes off
- the end product is the molecules with a double bond plus the halogen and water
what is regioselectivity?
preferential elimination at one site over another
what is a germinal dihalide?
molecules with two halogens on the same side of a double bond
what are the properties of elimination reactions?
- regioselectivity
- stereoselectivity
what is stereoselectivity?
preferential formation of one isomer over the other
what happens when a non-bulky base is used in elimination reactions?
a Zaitef product is formed, when a halogen turns into a double bond at the most substituted carbon in a trans manner to reduce steric strain
what happens when a non bulky base is used in elimination reactions?
a Hoffman is formed when a halogen turns into double bonds and goes to the least substituted carbon
what type of reaction do alcohols favour?
substitution
what type of reaction do alkoxide favour?
elimination
what type of reaction do more substituted alkyl halides favour?
elimination
what type of reaction do molecules with a bulky base favour?
elimination
what type of reaction with a strong acid, heat and a weak nucleophile favour?
elimination reaction
