Download Halogenoalkanes

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Reactions of grignard reagents
Nucleophilic substitution reactions of halogenoalkanes
-
S ubstitution with OH ions (hydrolysis).
Reagents and conditions are…
-
CH3CH2Br + OH
→
heat and reflux with dilute aqueous sodium
hydroxide solution.
CH3CH2OH + Br
-
CH3CH2Br + CN
-
→
CH3CH2Br + NH3
CH3CH2Br + 2NH3
→
→
With carbonyl compounds (aldehydes and ketones).
CH3CH2-MgBr + CH3CHO
heat and reflux with potassium cyanide in ethano l.
CH3CH2CN + Br
-
+
+
CH3CH2-CH(CH3)-O MgBr + H Cl
-
-
S ubstitution with NH 3 (aminolysis).
Reagents and conditions are…
Because grignard reagents are prepared in-situ, other substances are usually added to them
in dry ether. Gases, such as methanal and carbon dioxide, can be bubbled through. All the
reactions occur in two stages, with dilute aqueous acid such as HCl being added in the
second stage.
S ubstitution with CN- ions (cyanolysis).
Reagents and conditions are…
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heat in a sealed tube with excess concentrated
ammonia in ethanol.
CH3CH2NH2 + HBr or…
CH3CH2NH2 + NH 4Br
-
→
CH3CH2 -CH-O- MgBr+
CH3
→
CH3CH2 -CH-OH + MgBrCl
CH3
With CO2
CH3CH2-MgBr + CO2
-
+
-
+
CH3CH2-CO2 MgBr + H Cl
-
+
→
CH3CH2 -CO2 MgBr
→
CH3CH2 -CO2H
Note that this reaction is very exothermic so the solution must be cold, or dry ice (solid
CO2 at –78o C used instead).
Why is water not used as a solvent for the reactions with NH3 and CN ?
-
Both species are basic and react with water to give OH ions, so some of the halogenoalkane is
made into an alcohol instead: NH3 + H 2 O → NH 4+ + OH-
Formation of grignard reagents
Reactions of grignard reagents cont…
Grignard reagents can be formed from both alkyl and aryl halogeno compounds.
Reagents and conditions are…
gently warm the halo-compound with magnesium
turnings in dry ether, using a crystal of iodine if
necessary.
C 6H5 -Br + Mg
→
C 6H5 -MgBr
CH3CH2-Br + Mg
→
CH3CH2-MgBr
How and why are the conditions kept anhydrous?
Grignard reagents react with water and it inhibits their formation. Calcium chloride guard
tubes are attached to the apparatus (reflux condenser) and the halo-compound and they are
dried beforehand.
What is the purpose of the iodine crystal?
To initiate the reaction.
How does the reaction depend on the choice of halogen atom?
Bromo and iodo compounds are better choices as the C-Halogen bonds are longer and weaker,
making the compounds more reactive.
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With epoxy ethane.
O
CH3CH2-MgBr + CH2-CH2
-
+
+
CH3CH2-CH2-CH2 -O MgBr + H Cl
-
-
+
→
CH3CH2 -CH2-CH2-O MgBr
→
CH3CH2 -CH2-CH2-OH + MgBrCl
Notes
These reactions are classified as nucleophilic addition.
When methanal (CH2 O) is added, a primary alcohol with one extra carbon atom is
formed.
When other aldehydes are used (e.g. ethanal), secondary alcohols are formed.
When ketones are used (e.g. pro panone), tertiary alcohols are formed.
When CO2 is used, a carboxylic acid with one extra carbon is formed.
When epoxy ethane is used, a primary alcohol is formed with two extra carbons.
Because grignard reagents are prepared in-situ, yields of subsequent reactions are high as
there are no mechanical losses as a result of isolating the grignard reagent. Grignard
reagents can be used instead of toxic reagents such as KCN in order to introduce an extra
carbon atom, usually in fewer steps, which also improves yield.
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