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Organic Chemistry AS Level ~☆Sabrina Belfi ♡♡♡ > Chapter 16: Halogenoalkanes > Flashcards

Chapter 16: Halogenoalkanes Flashcards

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1
Q

Definition of nucleophilic addition

A

The mechanism of the reaction in which a nucleophile attacks the carbon atom in a carbonyl group and adds across the C=O bond, e.g. aldehydes, or ketones reacting with hydrogen cyanide.

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2
Q

Definition of SN1 mechanism

A

The steps in a nucleophilic substitution reaction in which the rate of the reaction (which is determined by the the slow step in the mechanism ) involves only the organic reactant, e.g. In the hydrolysis of a tertiary Halogenoalkane

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3
Q

Definition of SN2 mechanism

A

The steps in a nucleophilic substitution reaction in which the rate of the reaction (which is determined by the slow step in the mechanism) involves two reacting species, e.g. In the hydrolysis of a primary halogenoalkane.

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4
Q

Name 4 halogens in group 17 that we work with

A
  1. Fluorine
  2. Chlorine
  3. Bromine
  4. Iodine
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5
Q

The simplest Halogenoalkanes, whose molecules contain just 1 halogen atom will have the general formula …..

A

CnH2n+1 X

Where X is an halogenoalkane

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6
Q

What are the 3 nucleophilic substitution reactions of halogenoalkanes?

A
  1. Substitution reactions with aqueous alkali, OH-(aq)
  2. subtitution with cyanide ions, CN- (in ethanol )
  3. Substitution with ammonia , NH3 (in ethanol)
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7
Q

What 3 reactions do we deal with Halogenoalkanes?

A
  1. Nucleophilic substitution
  2. Hydrolysis
  3. Elimination
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8
Q

Substitution reactions with aqueous alkali, OH-(aq) of halogenoalkanes :

  1. What is needed for it to take place ?
  2. What atom is replaced by what?
  3. What organic product is formed ? And other product
  4. What acts as a nucleophile and how so ?
A
  1. NaOH (aq) to provide the hydroxyl group
  2. The Halogen atom in the halogenoalkane is replaced by the OH- group
  3. An alcohol is formed and a Na-halide e.g. NaBr
  4. The OH-(aq) acts like a nucleophile, because it is donating a pair of electrons to the carbon atom bonded to the halogen in the halogenoalkane (that’s how it attaches )
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9
Q

Silver nitrate can be used to test for halide ions, what colours are given by aqueous :

  1. Chlorides
  2. Bromides
  3. Iodides
A
  1. Cl = white ppt. (Silver chloride )
  2. Br = cream ppt
  3. I = yellow ppt.
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10
Q
  1. Substitution with cyanide ions , CN- (in ethanol ) :
  2. What acts as the nucleophile?
  3. What are the two conditions ?
  4. What is replaced in the molecule?
  5. Name of the product formed
A
  1. The cyanide , CN- Ion acts as the nucleophile
  2. Conditions :
    ● solution of potassium cyanide KCN , in ethanol ( known as ethanolic)
● heat under reflux
  3. The Halogen is replaced by the CN
  4. The name is the (alkane)nitrile e.g. propanenitrile
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11
Q
  1. Substitution with ammonia, NH3 (in ethanol ):
  2. What is the nucleophile
  3. What 3 conditions have to be met
  4. What product forms
A
  1. The ammonia molecule
  2. Conditions :
    ● heat
    ● pressure
    ● excess ammonia
  3. An amine forms
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12
Q

There are 2 possible mechanisms that can operate in the nucleophilic substitution reactions of halogenoalkanes, namely :

A
  1. Mechanism for primary halogenoalkanes (SN2) ~ formation of partial bonds
  2. mechanism for tertiary Halogenoalkanes (SN1)~ no formation of partial bonds
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13
Q

Broad details of the mechanism of nucleophilic substitution in halogenoalkanes .

  1. What does the nucleophile attack ?
  2. Explain what a nucleophile is and what it is attracted to
  3. The carbon - halogen bond is polarised, why?
  4. The Halogen is replaced by what in the substitution reaction?
A
  1. The nucleophile attacks the carbon -halogen bond
  2. Nucleophiles are electron pair donors and are attracted to electron deficient atoms
  3. The C-halogen bond is polar because the halogen is more electronegative than the carbon , C= s+ and X= s-
  4. The Halogen is replaced by the nucleophile
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14
Q

The mechanism of nucleophilic substitution in halogenoalkanes is determined by….

A

The structure of the halogenoalkane involved in the reaction

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15
Q

Mechanism of nucleophilic substitution in halogenoalkanes;
The mechanism for primary halogenoalkanes ( SN2) ~ Formation of partial bonds :

  1. What is a primary halogenoalkane
  2. Decibels the 3 steps in the mechanism of nucleophilic substitution in primary halogenoalkanes using bromoethane and HÖ- as the nucleophile
A
  1. A primary halogenoalkane is a molecule that contains one halogen atom bonded to a Carbon, which itself is bonded to 1 alkyl group
  2. Step 1:the ÖH- Ion donates a pair of electrons to the s+ Carbon
HÖ-  ⤵    
          H   H
           I    I
      H-C -C -H
           I    I
          Br  H

Step 2: a covalent bond forms between the OH and C atom . At the same time the C-Br bond is breaking heterolytically

HO ⚋⚋      H  H 
                       I    I 
                H-  C - C-H 
                       ⚋ I 
                       Br  H        
 dotted lines means bond breaking or making , in wrong direction because couldn't fine a correct symbol

Step 3: a Br- is left and the OH group is attached and an alcohol is formed :

        H    H
         I      I 
   H -C  -  C -H       +     Br-
         I       I
       OH    H
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16
Q

The mechanism for tertiary Halogenoalkanes
(SN1){ s -substitution, N- nucleophilic 1- the rate }:

  1. What is a tertiary Halogenoalkane
  2. This is a two step mechanism. Step 1 , step 2 ?
A
  1. A tertiary Halogenoalkane is when the carbon atom bonded to the halogen is also bonded to 3 alkyl groups

2.

 Step 1 :slow step , the breaking of the C-halogen bond which forms a tertiary carbocation 
         CH3
          I
H3C-C (s+)- Br (s-)
          I   ↪
         CH 3

Step 2: fast step, this tertiary cation is immediately attacked by the ÖH- Ion and the Br atom is substituted

           CH3
               I
     H3C-C+  ⤴ ÖH-              +     Br- 
               I
              CH3

End product is the OH is attached and the Br- is left by itself

17
Q

What is the difference in the products of a reaction of a halogenoalkane with NaOH (aq) or NaOH (ethanol)?

A

●NaOH (aq) forms an alcohol because nucleophilic substitution occurs

●NaOH (ethanol) forms a alkene because an elimination reaction occurs

18
Q

Elimination reactions of halogenoalkanes :

Facts: The halogen halide e.g. HBR is eliminated.
The ethanolic OH- acts as a Base accepting the H+ form the halogenoalkane.
The C-Br bond breaks heterolytically that’s why a Br - Ion is formed

What is the condition for an elimination reaction to occur to a halogenoalkane and what are the products formed when elimination takes place ?

A
  1. Condition = ethanolic sodium hydroxide , NaOH (ethanol)

Halogenoalkane + NaOH (ethanol ) ➡ alkene + water + Na-halide ( e.g. NaBr)

19
Q

4 uses of halogenoalkanes :

A
  1. Intermediates for making other useful substances
  2. Anaesthetics
  3. Non - stick lining of pans - poly (tetrafluoroethene)
  4. Ice skating blades
20
Q

What do CFCs refer to , and add 3 characteristics

A

Chlorofluorocarbons or chlorofluoroalkanes.

characteristics :

  1. Inert
  2. Not flammable
  3. Not toxic
21
Q

Give 3 uses of CFCs

A
  1. Aerosol propellants
  2. Solvents
  3. Refrigerants inside fridges
22
Q

What environmental issue do CFCs cause ? Plus a description of the ozone layer plus explain how it does what it does

A

The depletion of the ozone layer . The ozone layer protects the earth by absorbing harmful UV radiation arriving from the sun.

The UV light from the sun breaks the C -Cl bonds in the CFC molecules. This releases highly reactive chlorine free-radicals. These chlorine free radicals react with ozone molecules. A sequence of chain reactions happen and the the free radicals destroy the ozone molecules.

23
Q

What is a good substitute for CFCs?

A

HFCs hydrofluorocarbons

24
Q

The fastest nucleophilic substitution takes place with what halogenoalkanes

A

Iodoalkanes

25
Q

The slowest nucleophilic substitution reactions take place with what halogenoalkane

A

Fluoroalkanes

26
Q

Put the 4 halogenoalkanes in order of least reactive to most reactive

A

Least reactive ➡ most reactive
Fluoroalkanes, chloroalkanes , bromoalkanes, iodoalkanes

27
Q

We can investigate the rate of of hydrolysis using aqueous silver nitrate solution.

  1. What acts as the nucleophile
  2. What are 2 conditions for the hydrolysis of a halogenoalkane
  3. What are the products
A
  1. The water in the silver nitrate solution acts as the nucleophile
  2. Conditions = ● heat under reflux
    ● water
  3. Products = halide ion, an alcohol and an H + Ion
28
Q

What does heating under reflux do ?

A

It enables us to heat the reaction mixture without evaporating off the volatile organic compounds in the reaction flask

29
Q

Why is HFCs a good substitute for CFCs?

A

Because they are non-chlorinated compounds that break down more quickly once released in in the air because of the presence of hydrogen in their molecules, so they never rise in the atmosphere as far as the ozone layer.

Organic Chemistry AS Level ~☆Sabrina Belfi ♡♡♡ (5 decks)

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