15 haloalkanes Flashcards

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1
Q

Haloalkanes have the general formula

A

CnH2n+1X

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2
Q

draw primary seconday and tertiary

Classifying haloalkanes:

A

if the halogen is attached to C with 3 hydrogens, or 2 hydrogens and 1 alkyl group – it
is said to be PRIMARY

if the halogen is attached to C with 1 hydrogen and 2 alkyl group – it is said to be
SECONDARY

if the halogen is attached to C with 0 hydrogens and 3 alkyl group – it is said to be
TERTIARY

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3
Q

trend in bond enthalpy in c-halogen bonds down group

A

Bond enthalpy decreases

Weaker bonds
break more easily.

Meaning more of
them will break at
the same
temperature.

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4
Q

trend in bond polarity between C-halogen bond down the group

A

Bond polarity decreases

Greater
electronegativity makes
a larger δ-, meaning
the carbon will have a
larger δ+.

Attracts
nucleophiles more

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5
Q

Definition: Hydrolysis

A

a reaction with water or aq hydroxide ions that break a
chemical compound into two compounds

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6
Q

name? conditions?ragents?

haloalkane -> alcohol

A
  • hot aqueous hydroxide ions
  • reflux
  • hydrolysis
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7
Q

Hydrolysis is a ,……….. ……….. reaction.

A

Hydrolysis is a nucleophilic substitution reaction.

During hydrolysis, the halogen is replaced by the hydroxide ion. The negative
hydroxide ion is attracted to the δ+ carbon in the molecule, this is known as
nucleophilic attack.

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8
Q

The mechanism for nucleophilic substitution of haloalkane

A

The hydroxide attacks the central carbon from behind attack.

The lone pair electrons from the hydroxide form a dative covalent bond with the
carbon.

This carbon would have too many bonds so the carbon halogen bond breaks, as the
electrons are already nearest the δ- iodine.

When the carbon - iodine bond breaks heterolytically, the electrons go with the
iodine forming iodide, I- ions.

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9
Q

calculating rate of haloalkane hydrolysis

A
  • The precipitate that forms first indicates which haloalkane hydrolyses first. (AgI forms first, then AgBr, then AgCl)
  • Rate can be calculated by 1/time taken for the precipitate to appear.
  • In order for the test to be controlled, the test must be done with the same amounts, and similar haloalkanes, and at a constant temperature.
  • The rates of hydrolysis can be determined by heating the haloalkane with aqueous silver nitrate and ethanol.
  • The water in aqueous silver nitrate acts as the nucleophile
  • The silver nitrate is the test for the halides which are
    substituted during hydrolysis
  • The ethanol is a common solvent for the haloalkane and silver nitrate to mix.
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10
Q

the method

calculating rate of haloalkane hydrolysis

A

The procedure:

Measure an equal amount of each of the 1cm3 haloalkane
and place in a water bath at 50oC.

Put another tube of 2cm3 ethanol, 2cm3 water and 2cm3
silver nitrate into another tube and place in the water bath.

Once all the tubes have reached the same temperature add
an 0.5cm3 of the silver nitrate mixture to each of the 3
haloalkanes.

Start timing and record the times at which each of the
precipitates form.

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11
Q

x3

What affects the rate of hydrolysis:

A

1) Polarity
2) Bond enthalpy (energy)
3) classification

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12
Q

how does polarity effect the rate of hydrolysis

A

The C - Cl bond is the most polar of the 3
halogenoalkanes. This means that the δ+ carbon
in the C - Cl bond would attract nucleophiles
more readily. This means that it would give the
fastest reaction.
Electronegativity of the halogens decreases as you go down group 7. This means
that the C–Cl bond will be more polar than the C – Br bond which will be more polar
than the C–I bond.

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13
Q

how does bond enthalpy impact rate of hydrolysis of haloalkanes

A
  • The bond energies give us a good indication of the reactivity of the haloalkanes, and it is the bond enthalpy that predominantly determines the overall rate
  • The weakest would be expected to break first. The one that breaks first will form a
    precipitate with Ag+ first: The C - I bond is the weakest of the 3 halogenoalkanes.
    This means that the bond would break more readily.

This means that it would give the fastest reaction.

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14
Q

and why

how does classification impact rate of hydrolysis,

A

The classification of the haloalkane also plays a part. Tertiary is fastest, then secondary and
primary being relatively slower.

When a tertiary haloalkane reacts the mechanism actually occurs in two steps (SN2).

Step 1: the carbon-halogen bond cleaves leaving behind a stabilised tertiary
carbocation and the halide ion.

Step 2: the hydroxide ion attacks the carbocation to form the final organic product.

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15
Q

halogen containing polymers

PTFE - poly(tetrafluoroethane)

A

Used in the non – stick coating on saucepans and in electrical insulation.

C - F bonds are very strong and make it impervious to heat and chemical
attack hence it is used as a coating on pans.

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16
Q

halogen containing polymers

PVC – poly(chloroethene)

A

Used in drainpipes, window frames etc.

17
Q

structure of CFCs - chlorofluorocarbons

A

CFCs – e.g. Freon = molecular formula CF2Cl2

Tetrahedral arrangement around the central carbon.

Bond angle = 109.5o

18
Q

what is

Ozone

A

O3.

19
Q

The destruction of high level ozone allows harmful UV
radiation to reach earth.

A

Ozone filters out most of the harmful UV
radiation.

UV radiation is divided into a, b, and c.

C is the most harmful and is blocked out
completely by ozone.

A is the weakest and only a small
amount is absorbed by ozone.

  • gives you a tan and ages your
    skin prematurely
20
Q

Trouble with CFCs

A

CFC’s have a devastating effect on the ozone layer. The
ozone layer filters out harmful UV light which can cause skin
cancer.

CFC’s were used in refrigeration and aerosol propellants.
The stability of CFC’s has been the problem and the
concentration has slowly built up in the atmosphere.

In the stratosphere CFC’s absorb UV light forming Cl
radicals. It is these Cl radicals that break down ozone to O2

21
Q

Ozone depletion

CFC’s:

A

1) CFC’s:

UV light breaks the C – Cl bond releasing chlorine radical

CF2Cl2 → CF2Cl∙ + Cl ∙

This chlorine radical catalyses the decomposition of ozone with the chlorine radical coming out unchanged (and available for more ozone decomposition).

Cl∙ + O3 → ClO ∙ + O2 Step 1

ClO ∙ + O → Cl ∙ + O2 Step 2

22
Q

Ozone depletion

Nitrogen oxides:

A

Nitrogen oxides are formed by lightning strikes
and aircraft engines

∙ NO + O3 → ∙ NO2 + O2 Step 1

∙ NO2 + O → ∙NO + O2 Step 2

Overall

O(g) + O3(g) → 2O2(g)

23
Q
A