KA 3: synthesis

Question 1

what is bond fission?

Answer 1

the process of bond breaking

Question 2

what are the two types of bond fission?

Answer 2

-homolytic

-heterolytic

Question 3

describe homolytic fission

Answer 3

-two neutral radicals are formed

-occurs when each atom retains one electron from the sigma covalent bond

-bond breaks evenly

-occurs when non-polar covalent bonds are broken

Question 4

why are the products formed by homolytic fission unsuitable for organic synthesis?

Answer 4

homolytic fission tends to results in the formation of a very complex mixture of products

Question 5

describe heterolytic fission

Answer 5

-two oppositely charged ions are formed

-occurs when one atom retains both electrons from the sigma covalent bond

-the bond breaks unevenly

-occurs when polar covalent bonds are broken

Question 6

why is heterolytic fission better suited to organic synthesis?

Answer 6

result in far fewer products than reactions involving homolytic fission

Question 7

how can the movement of electrons during bond fission be shown?

Answer 7

curly arrow notation

Question 8

in reactions involving heterolytic bond fission, attacking groups are classified as…

Answer 8

nucleophiles or electrophiles

Question 9

describe curly arrow notation

Answer 9

single headed=single electron

double headed=pair of electrons

tail=source of electron(s)

head=destination of electron(s)

two single headed starting in the middle of a covalent bond=homolytic occurring

doubled headed starting in the middle of a covalent bond=heterolytic occurring

arrow with head pointing to space between two atoms= covalent bond will be formed between those two atoms

Question 10

what are nucleophiles?

Answer 10

-negatively charged ions or neutral molecules that are electron rich (molecules with lone pairs)

-attracted towards atoms bearing a partial (delta +) or full positive charge

-capable of donating an electron pair to form a new covalent bond

nulceophile=nucleus loving

Question 11

what are electrophiles?

Answer 11

-positively charged ions (e.g. carbocations) or neutral molecules that are electron deficient

-attracted towards atoms bearing a partial (delta -) or full negative charge

-capable of accepting an electron pair to form a new covalent bond

electrophile=electron-loving

Question 12

what is a carbOcation?

Answer 12

carbon with + charge

Question 13

what is a carbAnion?

Answer 13

carbon with - charge

Question 14

name the 8 reaction types

Answer 14

-substitution
-addition
-elimination
-condensation
-hydrolysis
-oxidation
-reduction
-neutralistion

Question 15

what are haloalkanes?

Answer 15

substituted alkanes in which one or more of the hydrogen atoms is replaced with a halogen atom

Question 16

describe monohaloalknes

Answer 16

-contanin only 1 H atom
-can be primary, secondary, tertiary
-take part in elimination reactions to form alkenes

Question 17

how are monohaloalkanes classed?

Answer 17

the number of alkyl groups attached to the carbon atom containing the halogen atom

Question 18

how do monohaloalkanes form alkenes via an elimination reaction?

Answer 18

using a strong base, such as potassium or sodium hydroxide in ethanol

Question 19

what can monohaloalkanes take part in nucleophilic substitution reactions with?

Answer 19

-aqueous alkalis to form alcohols
-alcoholic alkoxides to form ethers
-ethanoic cyanide to form nitrates (chain length increased by one carbon atom) that can be hydrolysed to carboxylic acids

Question 20

give three methods used to prepare alkenes

Answer 20

-dehydration of alcohols
-base-induced elimination of hydrogen halides from monohaloalkanes
-cracking

Question 21

describe the preparation of alkenes using the dehydration of alcohols

Answer 21

-requires a catalyst (hot alumin ox., conc sulfuric acid, conc phosphoric acid

-water is eliminated in the process

Question 22

describe the preparation of alkenes using base induced elimination of H-X from monohaloalkanes

Answer 22

-base used=potassium hydroxide (dissolved in ethanol)

Question 23

list the electrophilic addition reactions alkenes can undergo

Answer 23

-catalytic addition of hydrogen to form alkanes

-addition of halogens to form dihaloalkanes

-addition of hydrogen halides to form monohaloalknes

-acid catalysed addition of water to form alcohols

Question 24

describe the catalytic addition of hydrogen to form alkanes

Answer 24

-hydrogenation
-uses a heterogeneous catalyst e.g. nickel

Question 25

describe the addition of halogens to form dihaloaklkanes

Answer 25

-halogenation -the addition of cl2, br2 or i2 across double bond -bromine will lose its colour colour=>colourless product, so used for a test for alkenes

Question 26

describe halogenation in terms of polarity

Answer 26

the halogen is the electrophile as the Br-Br bond is very polarisable the approaching pi bond of an alkene induces a dipole to form

Question 27

describe the addition of hydrogen halides according to Markovinkov's rule, to form monohaloalkanes

Answer 27

-hydrohalogenation -addition of H-X across the double bond of an alkene there will be a favoured product when unsymmetrical alkenes are involved which we can predict using markovinkovs rule

Question 28

state markovnikov's rule

Answer 28

when a hydrogen halide or water is added to an unsymmetrical alkene, the hydrogen atom becomes attached to the carbon with the most hydrogen atoms attached to it already.

Question 29

describe acid-catalysed addition of water according to markovikovs rule to form alcohols

Answer 29

-hydration -addition of H-OH across double bond of an alkene -there will be a favoured product when unsymmetrical alkenes are involved which we can predict by using Markovnikov’s rule

Question 30

what is Sn1?

Answer 30

-occurs in a minimum of two steps -occurs via a trigonal planar carbocation intermediate

Question 31

what is Sn2?

Answer 31

-occurs in a minimum of two steps -occurs via a single five-centred, trignoal bipyramidal transition state

Question 32

how can the reactions mechanisms for Sn1 and Sn2 be represented?

Answer 32

using curly arrows

Question 33

Sn1 favoured by...

Answer 33

-tertiary haloalkanes- carbocation stabilised by alkyl groups (inductive stabilisation) -sterically hindered centres- no space for a five centred transition state

Question 34

Sn2 favoured by...

Answer 34

-primary haloalkanes- cant form a stable carbocation -less sterically hindered centres- lots of space for a five centred transition state

Question 35

describe what happens in the reactions of haloalkanes with aqueous alkalis to form alcohols

Answer 35

-alcohols can be oxidised to aldehydes or ketones

Question 36

describe the reactions of haloalkanes with ethanoic cyanide to form nitriles (which hydrolyse to carboxylic acids)

Answer 36

-if sodium or potassium cyanide in ethanol is heated under reflux with a haloalkane -the halogen is replaced by a CN group, this is usefulbecause the carbon chain gets extended by one