Hydrolysis of Butyl Bromide Isomers

Demonstration Experiment on Video

Hydrolysis of Butyl Bromide Isomers

Objective: Nucleophilic Substitution - S_N1 and S_N2

Peter Keusch

German version

Chemicals:
1-bromobutane (M = 137.03, d = 1.2785)
2-bromobutane (M = 137.03, d = 1.2585)
tert-butylbromide (2-bromo-2-methyl propane) 98 % (M = 137.03, d = 1.221)
bromothymol blue
ethanol 99 %
0.1 N NaOH

If the butylbromides are not perfectly colorless, then they must be distilled. (1-bromobutane, b.p. 101.6 °C; 2-bromobutane, b.p. 91.2 °C;
2-bromo-2-methyl propane, b.p. 73.3 °C). The butylbromides are to be stored in brown bottles.

Reagent solution: 740 mL of ethanol 99 %, 160 mL of dist. water, 0.032 g of bromothymol blue, 1.3 mL of 0.1 NaOH

Apparatus and glass wares:
heating mantle
thermometer
3 conical measures, graduated, 500 mL
round bottom flask 1.5 L
measuring cylinders 100 mL
measuring cylinde 500 mL
volumetric pipette 2 mL
pipette ball
3 snap-cap vials 20 mL
3 glass stirring rods

Hazards and safety precautions:

Butylbromide isomers are harmful if inhaled or swallowed. Eye and skin irritant.

Ethanol is highly flammable.

Safety glasses and protective gloves should be worn. Good ventilation required.

Experimental procedure:

The reagent solution is heated to 60°C using a heating mantle and a round bottom flask. Measured quantities of the butylbromide isomers are available in snap-cap vials:
Vial 1: 16 g of 1-bromobutane
Vial 2: 16 g of 1-bromobutane
Vial 3: 16 g of tert-butylbromide

300 mL of the reagent solution heated to 60 °C are placed in each of three conical measures.

1-bromobutane is poured into the third conical measure while stirring with a glass rod. 2-bromobutane and tertiary butylbromide, respectively, is added with stirring to the reagent solutions in the conical measures 2 or 1, respectively.

Results:

Video clip (Download RealPlayer .rm file)

The solution in conical measure 1 turns abruptly yellow. After some time, the solutions in the measures 2 and 3 appear green, before the color turns to yellow. 1-bromobutane (conical measure 3) reacts very slowly.

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 hydrolysis of the butyl bromide isomers is revealed by the indicator change from blue to yellow as hydrogen bromide is liberated in the reaction. The color change of the indicator permits the proof of the different reactivity of the alkyl halides. A heterolytic fission of the C-Br bond occurs.
The reactivity is rising in the order 1-bromobutane < 2-bromobutane < tertiary butylbromide. The three hydrolysis reactions have the same nucleophile and the same leaving group. Hence, the rates of the S_N reactions will depend on the different structures of the butylbromide isomers.

On the basis of mechanistical investigations can be proven:

· The hydrolysis of primary halides proceeds by an S_N2 mechanism. The reaction is bimolecular, i.e. two species are involved in the rate-determining step. The reaction rate depends on both the alkyl halide's (R - X) and the nucleophile's (Nu) concentration: R = k [ R - X ] [ Nu ].

The S_N2 (bimolecular nucleophilic substitution) involves rear-side attack of a nucleophile at the carbon atom, opposite to the leaving group being displaced. The incoming group replaces the leaving group in one step. Bond making and breaking occurs simultaneously. The "pentacoordinated" transition state of the S_N2 reaction is a trigonal bipyramid with the nucleophile and the leaving group located at the tops of the pyramids and the three remaining substituents located at the corners of the trigonal base. As the incoming nucleophile begins to bond with the carbon, the leaving group is departing with the bonding electrons. As a result of the mechanism, the three remaining substituents are rejected. The inversion of configuration resembles the way an umbrella turns inside out in a strong gust of wind (1). If the substrate under nucleophilic attack is chiral, this leads to an inversion of stereochemistry, called the "Walden Inversion".

· 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 (2). 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. This effect results in a mixture of two enantiomers (mirror image isomers).

· The hydrolysis mechanism of the secondary butylbromide depends very strongly on the reaction conditions.

References:
Rod Beavon S_N2 Nucleophilic substitution - bimolecular - Animation
Rod Beavon S_N1 Nucleophilic substitution unimolecular - Animation
Demonstration Experiment on Video Reaction of Butylbromide Isomers with Silver Nitrate
Demonstration Experiment on Video Hydrolysis of tertiary Butyl Halides

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