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ADV CHEM UNIT 3 > KA 3: synthesis > Flashcards

KA 3: synthesis Flashcards

1
Q

what is bond fission?

A

the process of bond breaking

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2
Q

what are the two types of bond fission?

A

-homolytic

-heterolytic

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3
Q

describe homolytic fission

A

-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

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4
Q

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

A

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

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5
Q

describe heterolytic fission

A

-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

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6
Q

why is heterolytic fission better suited to organic synthesis?

A

result in far fewer products than reactions involving homolytic fission

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7
Q

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

A

curly arrow notation

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8
Q

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

A

nucleophiles or electrophiles

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9
Q

describe curly arrow notation

A

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

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10
Q

what are nucleophiles?

A

-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

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11
Q

what are electrophiles?

A

-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

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12
Q

what is a carbOcation?

A

carbon with + charge

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13
Q

what is a carbAnion?

A

carbon with - charge

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14
Q

name the 8 reaction types

A

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

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15
Q

what are haloalkanes?

A

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

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16
Q

describe monohaloalknes

A

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

17
Q

how are monohaloalkanes classed?

A

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

18
Q

how do monohaloalkanes form alkenes via an elimination reaction?

A

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

19
Q

what can monohaloalkanes take part in nucleophilic substitution reactions with?

A

-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

20
Q

give three methods used to prepare alkenes

A

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

21
Q

describe the preparation of alkenes using the dehydration of alcohols

A

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

-water is eliminated in the process

22
Q

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

A

-base used=potassium hydroxide (dissolved in ethanol)

23
Q

list the electrophilic addition reactions alkenes can undergo

A

-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

24
Q

describe the catalytic addition of hydrogen to form alkanes

A

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

25
Q

describe the addition of halogens to form dihaloaklkanes

A

-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

26
Q

describe halogenation in terms of polarity

A

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

27
Q

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

A

-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

28
Q

state markovnikov’s rule

A

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.

29
Q

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

A

-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

30
Q

what is Sn1?

A

-occurs in a minimum of two steps

-occurs via a trigonal planar carbocation intermediate

31
Q

what is Sn2?

A

-occurs in a minimum of two steps

-occurs via a single five-centred, trignoal bipyramidal transition state

32
Q

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

A

using curly arrows

33
Q

Sn1 favoured by…

A

-tertiary haloalkanes- carbocation stabilised by alkyl groups (inductive stabilisation)

-sterically hindered centres- no space for a five centred transition state

34
Q

Sn2 favoured by…

A

-primary haloalkanes- cant form a stable carbocation

-less sterically hindered centres- lots of space for a five centred transition state

35
Q

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

A

-alcohols can be oxidised to aldehydes or ketones

36
Q

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

A

-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

ADV CHEM UNIT 3 (7 decks)

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