10.3 Types Of Organic Reactions

Question 1

What are the two types of mechanism for nucleophilic substitution reactions?

Answer 1

SN1 and SN2

Question 2

What does the type of mechanism that occurs depend on?

Answer 2

Whether the halogenoalkane is primary, secondary or tertiary.

Question 3

What does SN2 mean?

Answer 3

Bimolecular, two species involved in the rate determining step.

Question 4

What does SN1 mean?

Answer 4

Unimolecular, one molecule involved in the rate determining step.

Question 5

What do curly arrows show in the diagram of the mechanism?

Answer 5

The movement of a pair of electrons.

Question 6

What do fishhook arrows show in the diagram of free radical substitution?

Answer 6

The movement of one electron.

Question 7

What happens in the SN2 mechanism?

Answer 7

The molecule inverts as the nucelphile attacks so that id the original halogenoalkane is chiral the reaction will occur with the inversion of the configuration at the C attached to the halogen. This type of reaction is termed stereospecific (the stereochemistry can be predicted from that of the starting material).

Question 8

What type of halogenoalkanes undergo SN2 mechanism?

Answer 8

Primary halogenoalkanes.

Question 9

What type of halogenoalkanes undergo SN1 mechanism?

Answer 9

Tertiary halogenoalkanes.

Question 10

What happens in the SN1 mechanism?

Answer 10

The first step involves heterolytic fission of the C-Br bond the bond breaks so that both electrons go back at the same atom. Because the carbocation formed is planar the nucleophile can attack from either side in the second step and the reaction is not stereospecific if we start with an optically active halogenoalkane is a racemic mixture will be formed.

Question 11

What is the order of an SN1 mechanism?

Answer 11

First order overall.

Question 12

What is the order of an SN2 mechanism?

Answer 12

Second order overall.

Question 13

What is the effect of doubling the concentration of OH- on an SN1 mechanism?

Answer 13

No effect as OH- is not in the rate equation rate = k[(CH3)3CBr], it is only involved in a fast step after the rate determining step.

Question 14

What is the effect of doubling the concentration of OH- on an SN2 mechanism?

Answer 14

Rate of reaction doubles as the reaction is first order with respect to OH-
rate = k[CH3CH2Br][OH-] OH- is involved in the rate determining step.

Question 15

What is important about the mechanism secondary halogenoalkanes undergo during nucleophilic substitution?

Answer 15

Via a mixture mechanisms, the more dominant mechanism will depend on the conditions.

Question 16

What is the effect of structure on the rate of SN2 nucleophilic substitution?

Answer 16

The rate of reaction for SN2 is: primary > secondary > tertiary

Question 17

Why is tertiary the slowest rate of reaction?

Answer 17

Mainly due to stern effect, more alkyl groups surrounding the central C in a halogenoalkane makes it more difficult for the nucleophile to attack the central C.

Question 18

What is the effect of structure on the rate of SN1 nucleophilic substitution?

Answer 18

The rate of reaction for SN1 is: tertiary > secondary > primary

Question 19

Why is primary the slowest rate of reaction?

Answer 19

Due to the stability of the intermediate carbocation. Alkyl groups have an electron releasing effect (positive inductive effect) so that they are able to stabilise a positive charged carbon atom to which they are bonded. More alkyl groups resist in a greater electron releasing effect and greater stabilisation of the carbocation.

Question 20

What is the effect of the halogen on the rate of nucleophilic substitution?

Answer 20

R-I > R-Br > R-Cl > R-F

For both mechanism the rate determining step involved breaking the carbon halogen bond. The C-I bond is weakest therefore the reaction will be fastest for an iodoalkane.

Question 21

Why is OH- a better nucleophiles than water?

Answer 21

Because OH- is negatively charged and will be more strongly attracted to the positive C. Therefore the rate of an SN2 reaction is slower when OH- is replaced when water as a nucleophile.

Question 22

Why will changing the nucleophile have no effect on an SN1 reaction?

Answer 22

The nucleophile is only involved in the mechanism after the rate determining step, the nucleophile is not in the rate equation.

Question 23

How are polar solvents classified?

Answer 23

Protic solvents - have H joined to N or O and can participate in hydrogen bonding, such as water, ethanol or ammonia.
Aprotic solvents - do not have H joined to N or O and cannot participate in hydrogen bonding, these solvents do not acts as proton donors eg propanone.

Question 24

Why are SN1 mechanisms favoured by protic polar solvents?

Answer 24

Because solvent molecules are able to solvate both positive and negative ions which are generated in the reaction.

Question 25

Why are SN2 mechanisms favoured by aprotic polar and non polar solvents?

Answer 25

Because these are not very good at solving the negatively charged nucleophile this means that the nucleophile is not surrounded by the solvent molecules and is better able to get in and attack the positive carbon atom. For the same reason SN2 reactions are favoured by non polar solvents.

Question 26

What is an electrophile?

Answer 26

Species which is attached to regions of high electron density and accepts a pair of electrons to form a covalent bond. Electrophiles are lewis acids.

Question 27

How do alkenes react in an electrophilic addition mechanism with a halogen?

Answer 27

A halogen is non polar but as it approached the C=C which is a region of high electron density a dipole is induced in the halogen (positive is closer tot he C=C). With inter halogen compounds such as I-Cl consider the difference in the electronegativity to work out which is positive and attacks the C=C.

Question 28

How do alkenes react in an electrophilic addition mechanism with an interhalogen?

Answer 28

With inter halogen compounds such as I-Cl consider the difference in the electronegativity to work out which is positive and attacks the C=C.

Question 29

How do unsymmetrical alkenes react in an electrophilic addition mechanism with a hydrogen halide or interhalogen compounds?

Answer 29

More than one product can be formed depending on which way the atoms add across the double bond.

Question 30

What rule can be used to predict the major product from the electrophilic addition?

Answer 30

Markovnikov's rule - When H-X adds to the double bond of an alkene the H atom becomes attached to the C atom that has the larger number of H atoms already attached.

Question 31

How can Markovnikov's rule be explained?

Answer 31

This can be explained in terms of the stability of the carbocation intermediates formed. The secondary carbocation is more stable than the primary carbocation because there are more electron releasing ally groups attached tot he positively charged C in the secondary carbocation.

Question 32

What's important about inter halogen compounds with this rule?

Answer 32

The less electronegative halogen atom behaves as an H atom as it is positive and adds to the C atom with more hydrogens already attached.

Question 33

What reaction does benzene undergo?

Answer 33

Electrophilic substitution reactions.

Question 34

Why is substitution more favourable than addition for benzene?

Answer 34

The ring of six delocalised electrons is a region of high electron density and attracts electrophiles. Substitution is more favourable than addition as the more stable delocalised aromatic system is preserved.

Question 35

When does benzene undergo a nitration reaction?

Answer 35

In the presence of mixture of concentrated nitric and sulphuric acid.

Question 36

What are reducing agents to reduce aldehydes, ketones and carboxylic acids?

Answer 36

NaBH4 sodium borohydride - for aldehydes and ketons. | LiAlH4 lithium aluminium hydride - for carboxylic acids.

Question 37

How are aldehydes reduced?

Answer 37

To primary alcohols.

Question 38

How are ketones reduced?

Answer 38

To secondary alcohols.

Question 39

What kind of reducing agent is needed for carboxylic acids?

Answer 39

The stronger reducing agent LiAlH4

Question 40

What are carboxylic acids reduced to?

Answer 40

Primary alcohols - the reduction cannot be stopped at the aldehyde stage.

Question 41

How can nitrobenzene be reduced?

Answer 41

To phenylamine in a two stage reaction using a reducing agent such as tin and concentrated HCL.