Hydrolysis of tertiary Butyl Halides

Demonstration Experiment on Video

Hydrolysis of tertiary Butyl Halides

Objectives: Nucleophilic Substitution - S_N1, Effect of the Leaving Group on Rate

Peter Keusch

German version

Chemicals:
2-chloro-2-methylpropane 99 % (M.W. = 92.57, d = 0.851)
2-bromo-2-methylpropane 98 % (M.W. = 137.03, d = 1.221)
ethanol 96 %
0.2 % solution of bromothymol blue in EtOH 96 %
2 N NaOH

If the butyl halides are not perfectly colorless, then they must be distilled. (2-chloro-2-methylpropane, b.p. 51 - 52 °C;
2-bromo-2-methylpropane, b.p. 73.3 °C). The butyl halides should be stored in brown bottles.

Test solution: 800 mL of 96 % ethanol + 6.6 mL of 0.2 % solution of bromothymol blue in ethanol 96 % + 6.6 mL of 2 N NaOH + 200 mL of H₂O

Glass wares:
	2 conical measures, graduated, 500 mL
	2 glass stirring rods
	beaker 1500 mL
	2 snap-cap vials 20 mL
	2 pipettes 10 mL, graduated in 0.1 mL
	2 graduated cylinder 500 mL
	pipette bulb

Hazards and safety precautions:

tertiary butyl halides are harmful if inhaled or swallowed. Eye and skin irritant.

Ethanol is highly flammable.

Safety glasses, protective gloves and good ventilation required.

Experimental procedure:

400 mL of the test solution heated to 60°C are poured into each of two conical measures. Then, 2-chloro-2-methylpropane is added to the solution in conical measure 2 while stirring. Afterwards the solution in conical measure 1 is mixed with 2-bromo-2-methylpropane.

conical measure 1	2-bromo-2-methylpropane 5.6 mL = 50 mmole
conical measure 2	2-chloro-2-methylpropane 5.5 mL = 50 mmole

Results:

Approximately 20 seconds after the addition of 2-bromo-2-methylpropane the solution in measure 1 turns abruptly yellow. The solution of bromothymol blue in conical measure 2 remains unchanged.

Video clip (Download RealPlayer .rm file)

Discussion:

· Bromothymol blue (transition range: pH 6.0-7.8) is an acid-base indicator that appears blue in an alkaline (base) medium, green in neutral, and yellow in an acidic solution. The solvolysis of the tertiary alkyl halides is revealed by the indicator change from blue to yellow as hydrogen halide is liberated in the reaction.

· The solvolysis of the tertiary butyl halides in water takes place by anS_N1 mechamism . The reaction is unimolecular - only one species is involved in the slow step of the reaction. The reaction rate depends only on the concentration of the alkyl halide (R - X), not the nucleophile: R = k [ R - X ].

In an S_N1 reaction the key step is the loss of the leaving group to form the intermediate carbocation. This step is the slow, rate determining step of the reaction. The carbocation is then attacked by a nucleophile in a fast second step to form the product. The more stable the carbocation is, the easier it is to form, and the faster the S_N1 reaction will be. The planar, trigonal carbocation may be attacked equally well from either side by a nucleophile. As a consequence, an S_N1 reaction leads to a racemization, in which both retention and inversion of configuration at a chiral center occur to the same extent. Optically active tertiary haloalkanes produce a mixture of two enantiomers (mirror image isomers).

· In the rate-determining step of S_N1 reactions, the alkyl halide (R - X) is cleaved into a positively charged carbocation and a negatively charged leaving group. The reaction rate depends not only on the polarity of the solvent and on the stability of the carbocation, but also on the stability of the leaving group. The more stable the leaving group is, the more easily the C - X bond is also cleaved, the higher the reaction rate is. Conjugated bases of strong acids are good leaving groups. The experiment above shows that bromide ion is a better leaving group than the chloride ion. Bromide is a weaker base than chloride. The weaker base is more stable and thus more easily formed.

Relative hydrolysis rateg of R - X (R = tertiary alkyl group)
X = I > Br > Cl

Reference:
Microscale Projection Experiments Hydrolysis of tertiary Butyl Halides
Computer-Interfaced Experiments Kinetics: Hydrolysis of tertiary Butyl Halides - First Order Reaction
Computer-Interfaced Experiments Kinetics: Hydrolysis of Tertiary Butyl Chloride - First Order Reaction
Rod Beavon S_N1 Nucleophilic Substitution unimolecular - Animation
S_N1 Mechanism / S_N2 Mechanism

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