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chemistry > * Topic 6 Part 2 - Halogenoalkanes + alcohols > Flashcards

* Topic 6 Part 2 - Halogenoalkanes + alcohols Flashcards

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1
Q

How can halogenoalkanes be classified by and how is this decided

A

Halogenoalkanes can be classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group.

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2
Q

What types of reactions can halogenoalkanes undergo

A

Substitution or elimination

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3
Q

What is a nucleophilic substitution reaction

A

swapping a halogen atom for another atom or groups of atoms

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4
Q

What is a nucleophile

A

electron pair donator e.g. :OH-, :NH3, CN-

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5
Q

What does :Nu represent

A

:Nu represents any nucleophile – they always have a lone pair and act as electron pair donators

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6
Q

What do the nucleophiles attack in a nucleophilic substitution mechanism

A

The nucleophiles attack the positive carbon atom

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7
Q

Why does the carbon have a small positive charge in the first step of a nucleophilic substitution reaction

A

The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen

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8
Q

What do the curly arrows in mechanisms represent

A

We use curly arrows in mechanisms (with two line heads) to show the movement of two electrons

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9
Q

Describe the movement of a curly arrow in a mechanism (where does it begin and end)

A

A curly arrow will always start from a lone pair of electrons or the centre of a bond

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10
Q

What does the rate of nucleophilic substitution reactions depend on

A

The rate of these substitution reactions depends on the strength of the C-X bond
The weaker the bond, the easier it is to break and the faster the reaction.

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11
Q

Which halogenoalkane is the fastest to substitute and which is the slowest

A

The iodoalkanes are the fastest to substitute and the fluoroalkanes are the slowest. The strength of the C-F bond is such that fluoroalkanes are very unreactive

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12
Q

What is hydrolysis

A

Hydrolysis is defined as the splitting of a molecule ( in this case a halogenoalkane) by a reaction with water

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13
Q

Comment on the reactivity of water as a nucleophile

A

Water is a poor nucleophile but it can react slowly with halogenoalkanes in a substitution reaction

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14
Q

Give the equation (using X as the halogen) of the hydrolysis of a halogenoalkane

A

CH3CH2X + H2O -> CH3CH2OH + X- + H+

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15
Q

What test can be used to compare the reactivity of the different halogenoalkanes

A

Aqueous silver nitrate is added to a halogenoalkane and the halide leaving group combines with a silver ion to form a silver halide precipitate.
The precipitate only forms when the halide ion has left the halogenoalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different halogenoalkanes.

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16
Q

Which halogen forms the weakest C-X bond in a halogenoalkane

A

Iodine

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17
Q

Halogenoalkane -> alcohol

Reagent
Conditions
Mechanism
Role of reagent
Example - 1-bromopropane

A

Reagent - potassium (or sodium) hydroxide
Conditions - In AQUEOUS solution; heat under reflux
Mechanism - Nucleophilic substitution
Role of reagent - Nucleophile, OH-

1-bromopropane +KOH -> propan-1-ol + KBr

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18
Q

Why is it important for the reaction of a halogenoalkane to an alcohol that the conditions be aqueous

A

If the solvent is changed to ethanol an elimination reaction occurs

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19
Q

Why is OH- a stronger nucleophile than water

A

The OH– is a stronger nucleophile than water as it has a full negative charge and so is more strongly attracted to the Cδ+

20
Q

What is the difference between SN1 and SN2

A

SN1 - nucleophilic substitution mechanism for tertiary halogenoalkanes

SN2 - nucleophilic substitution mechanism for (primary) halogenoalkanes

21
Q

Why is SN1 able to occur in tertiary halogenoalkanes and not in primary halogenoalkanes

A

Tertiary halogenoalkanes undergo this mechanism as the tertiary carbocation is stabilised by the electron releasing methyl groups around it. (See alkenes topic for another example of this). Also the bulky methyl groups prevent the hydroxide ion from attacking the halogenoalkane in the same way as the mechanism above

Primary halogenoalkanes do not do the SN1 mechanism because they would only form an unstable primary carbocation.

22
Q

What are the steps of SN1 in 2-bromo-2-methylpropane

A

The Br first breaks away from the halogenoalkane to form a carbocation intermediate

The hydroxide nucleophile then attacks the positive carbon

23
Q

Draw the SN1 mechanism for a tertiary halogenoalkane (eg. 2-bromo-2-methylpropane)

A
24
Q

Draw the SN2 mechanism for a primary halogenoalkane (eg. 1-bromoethane)

A
25
Q

Halogenoalkane -> amine

Reagent
Conditions
Mechanism
Type of reagent

Ex of 1-bromopropane

A

reagent - NH3 dissolved in ethanol
Conditions - Heating under pressure in a sealed tube
Mechanism - Nucleophilic substitution
Type of reagent - Nucleophile; :NH3

1-bromopropane + 2NH3 -> Propylamine + NH4Br

26
Q

How to name amines

A

1-bromopropane + 2NH3 -> Propylamine + NH4Br

Naming amines:
In the above example propylamine, the propyl shows the 3 C’s of the carbon chain.
Sometimes it is easier to use the IUPAC naming for amines e.g. Propan-1-amine

27
Q

Draw the mechanism of 1-bromopropane to an amine

A
28
Q

Further substitution reactions can occur between the halogenoalkanes and the amines which leads to (1). (2) can be used to help minimise this

A

1 - a lower yield of the amine
2 - excess ammonia

29
Q

What further reactions can occur from the production of amines from halogenoalkanes

A

:NH3 ->(RX) :NH2R ->(RX) :NHR2 ->(RX) :NR3

30
Q

What is elimination

A

The removal of small molecules (often water0 from the organic molecule

31
Q

Halogenoalkane -> alkene

Reagent
Condition
Mechanism
Role of reagent
Ex with 1-bromopropane

A

Reagent - potassium (or sodium) hydroxide
Conditions - In ETHANOL; heat
Mechanism - Elimination
Role of reagent - Base, OH-

1-bromopropane + KOH -> propene +KBr +H2O

32
Q

How does the solvent impact the type of reaction (when reacting halogenoalkanes) aqueous vs alcoholic

A

aqueous - substitution
alcoholic - elimination

33
Q

With unsymmetrical secondary and tertiary halogenoalkanes, two (or sometimes three) different (1) isomers can be formed

A

1 - structural

34
Q

What can the structure of the halogenoalkane affect the degree of

A

The structure of the halogenoalkane also has an effect on the degree to which substitution or elimination occurs in this reaction.
Primary tends towards substitution
Tertiary tends towards elimination

35
Q

State some uses of halogenoalkanes

A

Halogenoalkanes have been used as refrigerants, fire retardants, pesticides and aerosol propellants.
Chloroalkanes and chlorofluoroalkanes can be used as solvents. CH3CCl3 was used as the solvent in dry cleaning

36
Q

What is a property of some halogenoalkanes that make them good at their uses

A

some halogenoalkanes have low flammability

37
Q

Why have many people stopped using halogenoalkanes in many uses

A

Many of these uses have now been stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the ozone layer.

38
Q

What is the general formula of alcohols

A

C(n)H(2n+1)OH

39
Q

What is the suffix ending of alcohols

A

-ol

40
Q

How do you name a compound that has an -OH group in addition to other functional groups that need a suffix ending

A

the OH can be named with the prefix hydroxy-

Eg. 2-hydroxypropanoic acid

41
Q

What is the bond angle of H-C-H and C-C-O bonds in alcohols and why

A

All the H-C-H bonds and C- H H C-O are 109.5 (tetrahedral shape), because there are 4 bond pairs of electrons repelling to a position of
minimum repulsion.

42
Q

What is the bond angle of H-O-C bonds in alcohols and why

A

The H-O- C bond is 104.5o (bent line shape), because there are 2 bond pairs of electrons and 2 lone pairs repelling to a position of minimum repulsion. Lone pairs repel more than bond pairs so the bond angle is reduced.

43
Q

What are the different types of alcohols

A

Primary, secondary and tertiary

44
Q

Give the equation of the combustion of alcohols in a (1) flame

A

1 - clean

CH3CH2OH + 3O2 -> 2CO2 + 3H2O

45
Q

What can the reaction of sodium and alcohols be used as a test for and state the equation of sodium reacting with ethanol

A

2CH3CH2OH + 2Na -> 2CH3CH2O-Na+ + H2

This reaction can be used as a test for alcohols

46
Q

State the observations of alcohol reacting with sodium

A
  • effervescence,
  • the mixture gets hot,
  • sodium dissolves,
  • a white solid is produced.

chemistry (9 decks)

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