carbon halogen bond forming reactions

Question 1

what is used to test for the presence alkenes and what is the result

Answer 1

bromine water, turns colourless (brown to colourless)

Question 2

what does homo and lumo mean

Answer 2

homo- filled pi orbital
lumo- empty pi orbital

Question 3

describe the mechanism of bromination of alkenes (testing for presence of alkene)

Answer 3

nucleophile= alkene
electrophile= Br2

alkene is electron rich, C=C pi bond is homo, bromine has low energy lumo, Br-Br bond is very weak

Br- approaches double bond in line with leaving group (max orbital overlap), results in inversion of stereochemistry to carbon that is attacked (SN2)

eg. ethene to dibromo-ethane

Question 4

what type of reaction is bromination of alkenes

Answer 4

sn2

Question 5

what happens if bromination is carried out in aqueous solvent with a base

Answer 5

makes synthetically useful epoxides, only trans diaxial (one is filled and one is dashed bond) bromohydrin (trans-2-bromocyclohexanol) can react/form epoxides

same axial bonded cyclohexanol cant react mechanistically

Question 6

what happens if bromination is carried out in aqueous alcohol solvent (Br- attacks first then alcohol is added)

Answer 6

solvent competes with bromide ion (even though OH is weaker nucleophile), higher conc solvent than Br-, solvent/alcohol attacks more substituted end of bromium ion

Question 7

what happens in bromination of alkene with HBr, difference between using a symmetrical vs unsymmetrical alkene

Answer 7

alkene reacts regioselectivity with HBr

symmetrical- no apparent regioselectivity observed

unsymmetrical- regioselectivity observed due to generation of most stable carbocation

Question 8

when adding HBr to alkene, how to decide which carbon gets the H

Answer 8

the hydrogen ends up on the carbon of a double bond that has more hydrogen atoms

Question 9

what happens when alkene reacts with HBr in the presence of dibenzoyl peroxide [(PhCO2)2]

Answer 9

produces the product with bromine atom substituted on carbon with most hydrogen atoms

Question 10

how can a benzoyl radical be formed from dibenzoyl peroxide

and what is the structure of both things

Answer 10

dibenzoyl peroxide has very weak O-O bond that can undergo homolytic cleavage to generate benzoyl radical

notes

Question 11

draw mechanism of how benzoyl radical is made from dibenzoyl peroxide

Answer 11

notes

Question 12

how can a bromine radical be formed

Answer 12

react benzoyl radical with HBr to give PhCO2H and Br radical

Question 13

what type of arrows are used in radical reactions

Answer 13

single headed

Question 14

what are radicals

Answer 14

neutral electron deficient species, more stable with more substituents

Question 15

how to make alkyl bromides from alkyl alcohols as alcohol is a poor leaving group

Answer 15

react alkyl alcohol with HBr, very harsh reaction conditions

Question 16

how to have milder conditions when converting alkyl alcohol to alkyl bromide

Answer 16

use reaction with an element that forms very strong bonds to oxygen like phosphorus and sulphur (both oxyphilic)

can use PBr3 (phosphorous tribromide) or SOCl2 (thionyl chloride)

very effective methods
inversion of stereochemistry since it is Sn2