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3.3 A level Organic Chemistry > 3.3.3 Halogenoalkanes > Flashcards

3.3.3 Halogenoalkanes Flashcards

1
Q

What are chlorofluorocarbons?

A

Chloroflurocarbons are halogenoalkane molecules with not hydrogen atoms but chlorine and fluorine atoms

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2
Q

Why is the ozone layer important?

A

Ozone (O3) is in the upper atmosphere and absorbs UV radiation from the sun, UV radiation can cause sunburn and skin cancer

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3
Q

Explain how ozone is formed?

A

Ozone is formed naturally when an oxygen molecule is broken down into two free radicals by UV radiation. The free radicals attack other oxygen molecules forming ozone.

O2 -UV-> O. + O.

then

O2 + O. -> O3

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4
Q

How is the ozone layer being destroyed by CFC’s?

A
  1. Chlorine free radicals (Cl.) are formed in the upper atmosphere when Cl-Cl bonds in CFC’s are broken down by ultraviolet radiation - e.g. CCl3F -UV-> CCl2F. + Cl.
  2. These free radicals are catalysts. They react with the ozone to form an intermediate (ClO.) and an oxygen molecule - e.g. Cl. + O3 -> O2 + ClO. or ClO. + O3 -> 2O2 + Cl.
  3. Since the chlorine is regenerated it can continue to attack other ozone molecules, so many will be destroyed
  4. The overall reaction is… 2O3 -> 3O2 … and Cl. is the catalyst
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5
Q

Why were CFC’s used in the first place, why were they later banned?

A
  • CFC’s are unreactive, non-flammable, and non-toxic. They used to be used as coolant gas in fridges, as solvents, and propellants in aerosols
  • In the 1970’s research by several different scientific groups showed that CFC’s cause damge to the ozone layer
  • The advantages of CFC’s weren’t worth the envriomental problems caused so they were banned
  • Chemists have developed safer alternatives to CFC’s which contain no chlorine e.g. HFC’s (hydrofluorocarbons) and hydrocarbons
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6
Q

What are halogenoalkanes?

A

A halogenoalkane is an alkane with at least one halogen atom inplace of a hydrogen - e.g trichloromethane

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7
Q

Why are halogenoalkanes prone to attack by nucleophiles?

A
  • Halogens are significantly more electronegative than carbon, therefore the bond is polar
  • The δ+ charge on the carbon makes it prone to attack by nucleophiles
    Halogens are prone to attack from nucleophiles such as OH-, CN- and NH3
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8
Q

What is a nucleophile?

A

A nucleophile is an electron pair donor

It donates a pair electons to electron deficient places

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9
Q

Describe nucleophillic substitution

A
  • The lone pair of electrons on the nucleophile attack the δ+ carbon in a polar bond, this causes the bond to break
  • The atom or functional group attached to the δ+ carbon leaves and a new bond between the δ+ carbon and the nucleophile is formed

The product of nucleophillic substition reactions depend of what the nucleophile is - e.g. OH-, CN-, or NH3

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10
Q

What does X stand for?

A

One of the halogens - e.g. F, Cl, Br, I

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11
Q

What does Nu stand for?

A

Nucleophile

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12
Q

Describe nucleophillic substitution between a hydroxide ion and bromoethane

A
  • The δ+ carbon attracts a lone pair of electrons from the OH- ion.
  • The C-Br bond breaks
  • The bromine leaves, taking both electrons to become :Br-
  • A new bond is formed between the carbon and the OH- ion, making an alcohol

This is sometimes called hydrolysis beacause the same reaction can occur with water

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13
Q

What does hydrolysis mean?

A

Splitting a molecule apart by reacting it with water

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14
Q

What are the possible reagents and conditions required for bromoethane to react with a hydroxide ion by nucleophillic substiution to produce an alcohol?

A

Warm aqueous sodium or potassium hydroxide

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15
Q

What reagents and conditions are required for a cyanide ion to react with a halogenoalkane by nucleophillic substitution to produce a nitrile

A
  • Warm under reflux
  • Ethanolic potassium cyanide
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16
Q

What reagents and conditions are required for a ammonia to react with a halogenoalkane by nucleophillic substitution?

A
  • Warm
  • excess ethanolic ammoinia
17
Q

Describe nuclearphillic addition between excess ethanolic ammonia and a halogenoalkane (bromine)

A
  1. The lone pair of electrons on the ammonia is attracted to the δ+ carbon in the C-X bond
  2. This causes the bond to break, Br leaves taking a lone pair with it to form :Br-
  3. The ammonia molecule forms a bond with the δ+ carbon, the ammonia on the halogenoalkane has a positve charge
  4. Next an ammonia removes a hydrogen atom from the ammonia bonded to the alkane, forming an ammonium ion (NH4+) and an amine group on the alkane
  5. The ammonium ion can react with the bromine ion to form ammonium bromide

Overall reaction: CH3CH2Br + 2NH3 -ethanol-> CH3CH2NH2 + NH4Br

The amine group on the product still has a ion pair so can act as a nucleophile, so can react with the halogenoalkane itself

18
Q

Why do halogenoalkanes have different speeds of reaction?

A
  1. The carbon-halogen bond strength (or enthalpy) decides reactivity, for any reaction to occur this bond must be broken
  2. The C-F bond is the strongest as it has the highest bond enthalpy, therefore fluoroalkanes undergo nucleophillic substitution reactions more slowly than other halogenoalkanes
  3. The C-I bond has the lowest bond enthalpy so iodoalkanes are substiuted more quickly
19
Q

Order halogenoalkanes in order of speed of reactivity

A

-Fastest (lowest bond enthalpy)-

  1. Iodoalkanes
  2. Bromoalkanes
  3. Chloroalkanes
  4. Fluoroalkanes

-Slowest (highest bond enthalpy)-

20
Q

What are conditions are required to eliminate a halogenoalkane with hydroxide ions to produce an alkene?

A
  • Warm under reflux
  • Ethanolic/ anhydrous (no water)
21
Q

How does elimination of a halogenoalkane using hydroxide ions work?

A
  1. OH- acts as a base and takes a proton, H+ from the carbon adjacent to the δ+ C, this makes water
  2. The adjacent carbon now has a spare electron, so it forms a double bond with the δ+ carbon
  3. To form the double bond, the bond between the δ+ C and the Br atom must be broken, the bromine atom leave as a :Br- ion
22
Q

Give the equation for when 2-bromopropane and potassium hydroxide react under reflux and ethanolic conditions

A

CH3CHBrCH3 + KOH -ethanol/reflux-> CH2CHCH3 + H2O + KBr

23
Q

Explain how changing the conditions reacting a halogenoalkane with a hydroxide together changes the type of reaction the products undergo

A

Nucleophillic substitution:
In aqueous conditions, OH- acts as a nucleophile

Elimination:
Anhydrous conditions, OH- acts as a base

If you use both, water and ethanol as solvents, both reactions happen and you get a mixture of products

3.3 A level Organic Chemistry (16 decks)

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