module 3

Question 1

Isomers?

Answer 1

Different compounds with the same molecular formula

Question 2

Constitutional isomers?

Answer 2

Isomers with a different order of attachment

Question 3

Stereoisomers \?

Answer 3

Same order of attachment of atoms, but different orientation of their atoms in space.

Question 4

Enantiomers?

Answer 4

Stereoisomers that are nonsuperimposable mirror images of each other.

Question 5

Diastereoisomers?

Answer 5

Stereoisomers that are not mirror images.

Question 6

How to identify E and Z isomers?

Answer 6

Assign priorities to the molecules on the left and right sides of a carbon double bond. If the highest priority atoms are on opposite sides of the molecule (top and bottom) the isomer is E (trans) and alternatively same side would be Z (cis).

Question 7

What Letter is assigned if an enantiomers configuration is clockwise?

Answer 7

R Configuration.

Question 8

What Letter is assigned if an enantiomers configuration is anti-clockwise?

Answer 8

S Configuration

Question 9

What is a homolytic bond cleavage?

Answer 9

ONE electron is transferred to each of the TWO atoms. The species generated (X. and Y. are radicals.

Question 10

What is a heterolytic bond cleavage?

Answer 10

TWO electrons are transferred from the bond to ONE atom. The species generated are charged intermediates.

Question 11

When simply bonded functional groups are attached, why is the c-x bond polar?

Answer 11

The X group is more electronegative that C.

The C atoms is electrophilic (seeking electrons).

Question 12

What are Nucleophiles?

Answer 12

Nucleophiles are species that can donate a pair of electrons to the positive carb and form a bond.

Question 13

Describe general nucleophilic substitution reactions?

Answer 13

If X is a good leaving group (C-X bond is weak) then when a nucleophile approaches the c-x bond will break and the nucleophile will form a new bond with the C atom.

Question 14

Describe general elimination reactions?

Answer 14

Occurs in slightly more complex molecules (more than one carbon atom. Nucleophiles can also act as bases. When it approaches it will remove a proton (Hydrogen atom) from the molecule, those electrons are transferred to the C atom resulting in the formation of a Pi bond (double bond).

Question 15

What is an Sn2 reaction?

Answer 15

Nucleophilic Substitution
X leaves and Nu approaches
One step reaction
Reactants - transition state - products

Question 16

Explain the transition state of an Sn2 reaction?

Answer 16

Effective collision has taken place
Corresponds to maximise free energy
Nu and X are both loosely bound to C
If activation energy is high the reaction will be slow

Question 17

What is a Sn1 reaction?

Answer 17

Nucleophilic substitution
X leaves, then Nu attaches
Two step reaction
Slow step - Transition state (Reaction intermediate) - fast step - deprotonation.

Question 18

Explain the slow step of an Sn1 reaction?

Answer 18

Removal of leaving group (independent of nucleophile)

Bond stretched to breaking point (transition state)

Question 19

Explain fast step in Sn1 reaction?

Answer 19

Addition of nucleophile

Question 20

Explain the Transition state of Sn1 reaction?

Answer 20

Carbocation produced from slow step acts as the reaction intermediate. It is very reactive and will react with any nucleophile and major product will be from from reaction with most abundant nucleophile.

Question 21

Stereochemistry of Sn2 reaction?

Answer 21

Nu approaches opposite to leaving group

Sn2 reaction is stereospecific

Question 22

Stereochemistry of Sn1 reaction?

Answer 22

Nu can approach and attach to a planar reaction intermediate (carbocation) from either pole.
Occurs with inversion and retention of stereochemistry.

Question 23

What types of molecules go via Sn2?

Answer 23

Methyl, Primary, Secondary (can do either)

Question 24

What types of molecules go via Sn!?

Answer 24

Secondary (can do either) , Tertiary

Question 25

What determines the rate of Sn1?

Answer 25

Can be fast if changes in transition state are stabilized | .

Question 26

What determines the rate of Sn2?

Answer 26

Can be fast if approach of nucleophile is not sterically hindered. Eg collision with C or too many groups attached to C.

Question 27

In both pathways what step does the loss of leaving group occur in?

Answer 27

Rate determining step.

Question 28

How do the two mechanisms differ in terms of leaving group requirements?

Answer 28

For Sn2 a strong Nu can make up for a poor leaving group but for Sn1 a good leaving group is essential.

Question 29

Can some compounds be converted into better leaving groups?

Answer 29

Yes, HO and RO under strong acidic conditions (protonation) because the +ve charge pulls the electrons in the c-o bond towards the oxygen, weakening the bond.

Question 30

Example of a good and poor leaving group?

Answer 30

``` OH = good H20 = poor ```

Question 31

What stabilizes the transition state in Sn1 reactions?

Answer 31

H20 being a polar solvent polar effect of C H bonding to L

Question 32

What determines Elimination reaction mechanisms?

Answer 32

base strength. Strong base = E2 Weak base = E1

Question 33

What is an E1 mechanism?

Answer 33

Elimination reaction Two step reaction Often co-occurs with Sn1 reactions

Question 34

Explain first step of E1?

Answer 34

Rate determining step Same as Sn1 generates are carbocation intermediate

Question 35

Explain second step of E1?

Answer 35

A base (H20 most abundant) removes H atom to form c=c bond.

Question 36

What is an E2 mechanism?

Answer 36

Elimination reaction | One step reaction

Question 37

Explain. step one of E2 mechanism ?

Answer 37

Concerted bond breaking and bond formation. Accessible H+ is removed Halide and Base involved in single rate determining step Must have strong base Possible for primary, secondary, and tertiary

Question 38

What is an Addition reaction?

Answer 38

Alkene carbocation doubled bond reacts with nucleophile and electrophile. Dissociation of reactant into Nu and E followed by Two step reaction.

Question 39

Explain first step of Addition reactions?

Answer 39

Slow step - addition of electrophile to the electron rich double bond perpendicular to the atoms plane. (Rate determining step)

Question 40

Explain Second step of Addition reactions?

Answer 40

Fast step - attack by nucleophile of the carbocation intermediate. (Carbocation intermediate will react with the most abundant nucleophile.)