JB CI 13.1 Halogenoalkanes

8/9/2019 JB CI 13.1 Halogenoalkanes

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CI 13.1 HalogenoalkanesAre man-made compounds with one or more halogen atoms ( F,Cl, Br , I) attached to a carbon atom. The attached halogen changesthe chemical properties of alkane chainsthey are veryunreactive , and so have been very useful to humans.

Naming halogenoalkanes (haloalkanes)(similar rules to naming alcohols, just add the halogen as a prefix):

halogens are in alphabetical order. lowest numbers possible are used.

CH 3CH 2CH 2Cl is 1-chloropropaneCH 3CHClCH 2Cl is 1,2-dichloropropaneCH 3CHBrCH 2CH 2Cl is 3-bromo, 1-chlorobutane.CH 3CHICHBrCH 2Cl 2-bromo,1-chloro,3-iodobutane2-bromo, 3-chloro, 1-iodopentaneCH 2ICHBrCHClCH 2CH 3

Physical properties of halogenoalkanes immiscible with water The bigger the halogen atom /the larger the number of

halogen atoms the higher the boiling point. Larger halogen atoms (Br or Cl ) cause greater environmental

damage than smaller halogen atoms (F); this is importantwhen designing replacements for CFCs.

Chemical reactions of halogenoalkanesCarbon halogen (C-Hal) bonds can break either homolytically or heterolytically .


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Homolytic Fission forms radicals eg when a halogenoalkaneabsorbs radiation of the right frequency.

H H

H C Cl + h v H C + Cl

H HChloromethane methyl radical chlorine radical

Shorthand is: CH 3-Cl + h v CH 3 + Cl (occurs in stratosphere).

Heterolytic fission is more common in lab conditions using polar solventssuch as ethanol or ethanol and water. The polar C-Hal bond can break,leaving a negative halide ion and positive carbocation .

CH 3 CH 3

CH 3 C Cl + h v CH 3 C+ + Cl -

H H2-chloro-2-methylpropane carbocation chloride ion(negatively charged substances may react with the positive carbocationcausing a substitution reaction).

Importance of reaction conditionsfor determining how bonds break

Eg. Bromoethane C-Br bonds break:

Heterolytically , forming ions when dissolved in a polar solvent(say a mixture of ethanol and water) BUT

Homolytically , in the gas phase at high temp. or when dissolved in anon-polar solvent , such as hexane.

Different halogens, different reactivity.All reactions with halogenoalkanes involve breaking the C-Hal bond. TheC-F bond is the strongest (bond enthalpy 467 kJmol -1) and therefore the


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hardest to break, whereas the C-I bond is relatively weaker (228 kJmol -1)and therefore easier to break. C-Hal bonds get weaker, and so morereactive, down group 7.Chloro compounds are fairly unreactive and remain in the troposphere

long enough to reach the stratosphere, where they react with and destroythe ozone layer.

Substitution reactions of halogenoalkanes

Halogenoalkanes can hydrolysed by hydroxide ions to form alcohols.

Eg. Bromobutane forms butanol:

CH 3 CH 2 CH 2 CH 2 Br + OH - CH 3 CH 2 CH 2 CH 2 OH + Br -

The C-Br bond is polar The oxygen atom on OH - is vely charged. + _C Br HO

The partial positive charge on the carbon atom attracts the negativelycharges oxygen of the hydroxide ion. A lone pair of electrons on the Oatom forms a bond with the C atom as the C __ Br bond breaks.

H H H H H H H H _ H __ C __ C __ C __ C __ Br H __ C __ C __ C __ C __ O __ H + Br

H H H H H H H H_

OH

Halogenoalkanes can give substitution reactions with hydroxide ionsand other NUCLEOPHILES . Nucleophiles can donate a pair of electronsto a positively charged carbon atom to create new covalent bonds.

Heterolytic fission results in IONS and not radicals.Curly arrows show the movement of electrons (full headedarrows for a pair of electronsunlike radical reactions).


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Some common nucleophiles:

Name Formula Structure showinglone pairsHydroxide ion OH - _

H __ O

Ethanoate ion CH 3COO - _ CH 3 __ C __ O

O

Ethoxide ion C 2H 5O-

_ CH 3CH 2 __ O

Water molecule H 2OO

H HAmmonia molecule NH 3

NH HH

Cyanide ion CN - _ N C

The carbon atom attacked by the nucleophile may be part of acarbocation and carry a full positive charge , or it may be part of aneutral molecule (as in the above example with bromobutane) and carry a

partial positive charge as a result of bond polarisation.

If X - represents a nucleophile, the nucleophilic substitution process is:

+

C Hal C __ X + Hal -

X


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Water as a nucleophile

Nucleophiles may be neutral or have negative charge, so long as it has alone pair of electrons which can form a bond to a carbon atom.Eg. Water has 2 lone pairs of electrons on the O atom. First it attacks thehalogenoalkane (bromobutane in this case):

H H H H H H H H _ H __ C __ C __ C __ C __ Br H __ C __ C __ C __ C __ O __ H + Br

H H H H H H H H H

OH H

The resulting ion loses H + to form an alcohol:

H H H H + H H H HH __ C __ C __ C __ C __ O H __ C __ C __ C __ C __ O __ H + H +

H H H H H H H H H

The overall equation for the reaction of water with a genera;halogenoalkane R __ Hal is:

R __ Hal + H 2O + R __ OH + H + + Hal -

Ammonia as a nucleophile

A lone pair of electrons on the N (similar to water) attacks thehalogenoalkane to produce an AMINE with an NH 2 group:

R __ Hal + NH 3 R __ NH 3+ Hal - R __ NH 2 + H + + Hal -


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Using nucleophilic substitution to make halogenoalkanes

Halogenoalkane + OH-

alcoholHalogenoalkanes can be made via the reverse reaction of making alcohols;the nucleophile is Hal -.

Eg. 1-bromobutane is made using a nucleophilic substitution reaction between butan-1-ol and Br - ions, in the presence of a strong acid .Ist step: H + ions bond to O atom on the alcohol:

H H H H H H H H +H __ C __ C __ C __ C __ O H __ C __ C __ C __ C __ O __ H

H H H H H H H H HH+

This gives the C atom to which the O is attached a greater partial positivecharge. It is now more readily attacked by Br - ions, forming bromobutane.

H H H H + H H H HH __ C __ C __ C __ C __ O H __ C __ C __ C __ C __ Br + H 2O

H H H H H H H H H_

Br

The overall equation for the reaction is:

CH 3CH 2CH 2CH 2OH + H + + Br - CH 3CH 2CH 2CH 2Br + H 2O

Activity A4.2Problems for 13.1 pages 303- 304 questions 1- 9.

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JB CI 13.1 Halogenoalkanes