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chemistry A - level > organic chemistry - halogenoalkanes and alkenes > Flashcards

organic chemistry - halogenoalkanes and alkenes Flashcards

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1
Q

what do halogenoalkanes contain?

A

at least one halogen atom in the compound. this could be F, Cl, Br, I

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2
Q

How are halogenoalkanes formed?

A

from alkanes in free radical substitution

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3
Q

when do the boiling points increase in halogenoalkane?

A

the significant increase in size of the halogen atoms (increase in Mr) causes greater van der waal forces which increase the boiling point

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4
Q

what bonds do halogen alkanes contain?

A

polar bonds

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5
Q

are halogens soluble in polar solvents (e.g. water?)

A

generally not soluble/ miscible in polar solvents

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6
Q

are halogens soluble in non-polar solvents (e.g. hexane or other hydrocarbons?)

A

they are soluble in non-polar solvents.
this is particularly true for longer chained molecules

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7
Q

what is a nucleophile?

A

electron pair donor

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8
Q

most common nucleophiles:

A
  • CN:
  • OH:
  • NH3
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9
Q

what’s nucleophilic substitution

A

changing functional group
a mechanism that shows how nucleophiles attack halogenoalkanes . produces alcohols or amines

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10
Q

conditions and reagents for nucleophilic substitution with NaOH

A
  • NaOH in aqueous solution
  • heat gently and reflux
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11
Q

conditions and reagents for nucleophilic substitution with KCN (potassium cyanide)

A
  • KCN in aqueous solution
  • heat gently and reflux
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12
Q

conditions and reagents for nucleophilic substitution with NH3 (ammonia (non-charged nucleophile))

A
  • concentrated ammonia in excess
  • heat
  • in a sealed container under pressure
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13
Q

how does NH3 act in nucleophilic substitution

A

NH3 is a base (it accepts H+ ions)

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14
Q

hydrolysis of halogenoalkanes

A

C-I is the easies to break compared to C-Br and C-Cl . therefore its hydrolyses fastest

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15
Q

what is the role of ethanol with halogenoalkanes

A

it dissolves halogenoalkanes and mixes with water, used in elimination reactions

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16
Q

What is elimination? And whats its products for halogenoalkanes?

A

A mechanism that involves the removal of an atom or group of atoms. Halogenoalkanes undergo elimination to form alkenes (C=C)

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17
Q

when does elimination occur?

A

when a halogenoalkane is heated to high temp under alcoholic conditions

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18
Q

how does the nucleophile act in an elimination reaction?

A

acts as a base and accepts a proton, removing a hydrogen atom form the molecule. this results in the elimination of the halide too producing a carbon= carbon

19
Q

The mechanism for elimination

A
  1. OH- acts as a base to remove H from an adjacent carbon (next to)
  2. Electrons in C-H bond, move to C-C bond to form an alkene
  3. C-Cl bond breaks to give electrons to the Cl
20
Q

What is the functional group of alkenes and what does this limit?

A

C=c
This double bond has limited rotation and this gives rise to stereoisomerism

21
Q

How can you work out if an alkene is Z or E

A

The “priority group” is chosen based on the proton (atomic) number of the adjacent atom

22
Q

Structure for Z

A

Two important parts are together

23
Q

Structure for E

A

Two important parts are opposite

24
Q

Alkenes
General formula
Functional group

A

Cn H2n
C = C

25
Q

Melting and boiling points of alkenes

A

They are low due to weak intermolecular forces , van der walls, between molecules
Similar to alkanes but a little higher

26
Q

Polarity of alkenes

A

Non-polar molecules as there’s no electronegativity difference between C’s

27
Q

Polymerisation of alkenes

A

where many monomers join to form long chained polymers. the double bond is broken to form a repeating unti
E.g. ethene -> poly(ethene)

28
Q

reaction conditions for the production of polymer chains

A
  • high pressure & temp - produces branched chain polymers with weak IMF
  • low pressure & temp - produces straight chain polymers with strong IMF
29
Q

What are polymers

A

unreactive hydrocarbon chains with multiple strong, non-polar covalent bonds.
though due to them being unreactive they are not biodegradable

30
Q

what is PVC

A

poly(chloroethene) is an addition polymer with waterproof properties because of the addition of plasticisers during the reaction

31
Q

Whats an electrophile

A

An electron pair acceptor

32
Q

what are common electrophiles?

A
  • HBr
  • Br2
  • H2SO4
33
Q

electrophilic addition

A

breaks the double bond and a carbocation forms. this is a carbon atom with 3 bonds so it has a + charge

34
Q

what can electrophiles be used to form?

A
  • alkyl hydrogensulphates
  • halogenoalkanes
35
Q

Definition of addition

A

Adding into a compound (loss of a double bond)

36
Q

Electrophilllic addition
Ethene plus H-Br

A

1) polar H-Br approaches e- rich C=C
2) electrons from C=C attack H (delta +) to form new C-H bond, breaking H-Br bond
3) this forms a carbocation (positively charged ion) and Br-
4) lone pair form Br- forms new bond to C+

37
Q

What is addition with asymmetric alkenes

A

When there are different “options” as to which carbon forms the first bond, and which has the positive charge

38
Q

How can you figure out what product is major or minor?

A

They are due to the different carbocations that form and their stability

Primary carbocation (1 carbon is adjacent) - least stable
Secondary carbocation (2 adjacent carbons)
Tertiary carbocation (3 adjacent carbons) - most stable

39
Q

How can a product be major or minor?

A

Because of the “inductive effect” of other carbons
Adjacent carbons push electron density towards the positive charge to stabilise it

40
Q

what does ozone do?

A

o3, forms a layer in our atmosphere and absorbs uv radiation. its becoming depleted due to CFC

41
Q

what do CFC’s do?

A

chloro-fluoro carbons absorb uv radiation, breaking down carbon- halogen bonds to form free radicals that can catalyse ozone depletion into oxygen

42
Q

what are we doing to prevent ozone depletion and global warming

A

CFC-free solvents are now being produced to prevent them entering the atmosphere

43
Q

what alternatives do we use instead of CFC

A

we use hydrofluorocarbons as they do not generate Cl radicals. they’re used in cool and refrigerants and propellants

44
Q

what does bromine water do when present with an alkene?

A

turns the solution from orange to colourless

chemistry A - level (16 decks)

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