Alkenes by Elimination

Abstract: The objective of this experiment is to successfully perform a dehydration of a 2-butanol and a dehydrohalogenation of 2-bromobutane to form the products 1-butene, trans-2-butene, and cis-2-butene. It was found that a dehydration of 2-butanol yielded 4.6% 1-butene, 67.3% trans-2-butene, and 28.1% cis-2-butene, and a dehydrohalogenation of 2-bromobutane yielded 19.1% 1-butene, 69.9% trans-2-butene, and 11.0% cis-2-butene.

Backround:

Gas Chromatography separates organic samples much in the same way as column chromatography. The only differences are that it uses a moving gas phase and a stationary liquid phase, and that the temperature of the gas system can be controlled. In a gas chromatograph the sample is shot in with a syringe and is immediately vaporized in a heated injection chamber. It is then introduced to a moving stream of gas called the carrier gas which sweeps the vaporized sample into a column filled with particles filled with liquid adsorbent. This column is usually filled with liquid that has a low vapor pressure and high boiling and is called the stationary phase. This phase is also usually coated onto a support material very evenly and packed into a tubing apparatus as evenly as possible and placed in the temperature controlled oven. When organic solutions are passed through the tubing van der Waals forces attract the nonpolar molecules especially if they have large molecular weights. Polar molecules can be attracted in many ways. Interactions include salt formation, coordination, hydrogen bonding, and even dipole-dipole. Through these interactions the molecules in the vaporized sample will separate accordingly. Finally at the end where the gases come out is a detector which generates a signal that is recorded on a strip chart recorder.

Elimination reactions are reactions that split a single reactant into two products. Elimination reactions can then be categorized as either an E1 (first order) or E2 (second order) reaction. In an E1 mechanism, a monomolecular elimination of water will result in an alkyl cation. In an E2 mechanism, a bimolecular reaction occurs in which water acts as a base, abstracting a â-hydrogen. The direction of the elimination reaction forming alkenes is typically governed by Zaitsev’s rule. Zaitsev’s rule states that the most highly substituted alkene will be formed preferentially, which basically states that the double bond folds towards the more substituted carbon.

Objective and Materials:

The materials for this experiment is listed on page 87-92(Organic Chemistry laboratory Manual, Haden/McNeil, 2003-04)

Procedure:

The procedure for this experiment is listed on page 87-92

(Organic Chemistry laboratory Manual, Haden/McNeil, 2003-04)

Data:

All data is in laboratory notebook

Calculations:

The following is a calculation for approximation of area of triangular peak:

Area = Height * ½ base

The following is a sample calculation of the area produced by 1-butene from 2-butanol

0.55cm^2 = 1.1cm * ½ (1cm)

The following is a calculation of percent chemical component(Given known order):

% component = Area of component / Total area of all components * 100

The following is a sample calculation of % 1-butene from 2-butanol

4.6% = 0.55cm^2 / 11.76cm^2 * 100

Results:

The following is a chart of retention time and percent product from 2-butanol:

Components% productRetention Time (s)

1-butene4.616

trans-2-butene67.316

cis-2-butene28.116

The following is a chart of retention time and percent product from 2bromobutane:

Components% productRetention Time (s)

1-butene19.118

trans-2-butene69.918

cis-2-butene1118

-It was found that trans-2-butene formed the most product in both cases.

Discussion:

The objective of this experiment is to successfully perform a dehydration of a 2-butanol and a dehydrohalogenation of 2-bromobutane to form the products 1-butene, trans-2-butene, and cis-2-butene. It was found that a dehydration of 2-butanol yielded 4.6% 1-butene, 67.3% trans-2-butene, and 28.1% cis-2-butene, and a dehydrohalogenation of 2-bromobutane yielded 19.1% 1-butene, 69.9% trans-2-butene, and 11.0% cis-2-butene. Trans-2-butene was found to be the overwhelming product in both cases. The following are the mechanisms for both of the reactions:

Dehydrohalogenation of 2-bromobutane:

1)Base (B:) attacks a neighboring hydrogen and begins to remove the H at the same time as the alkene double bond starts to form and the Br starts to leave.

2) Neutral alkene is produced when the C-H bond is fully broken and the Br has departed with the C-Br bond electron pair. Dehydration of a 2-butanol:

1)2 electrons from the oxygen atom bond to H+, yielding a protonated alcohol intermediate.

2)The carbon-oxygen bond breaks, and the 2 electrons from the bond stay with the oxygen, leaving a carbocation intermediate

3)2 electrons from a neighboring carbon-hydrogen bond form the alkene Ð bond, and H+ (a proton) is released.

In both of these reactions trans-2-butene is the overwhelming product yield. This is because in these E2 reactions, an electron pair from a neighboring C-H bond pushes out the leaving group on the opposite side of the molecule forming a anti-periplanar geometry (also known as trans stereochemistry).

A number of errors could have happened in this lab. First if the gas producing tube was put in the products tube too early some of the product could have been air (first 8mL is air in the tube). This would cause the gas chromatographer to give a reading of air as one of the components. A second error could have happened if the tubing was not removed from the water bath before cooling the reaction flask. This would could cause water to be sucked up into the reaction vessel, which contains concentrated acids which would then react with the water.