Question

During debromination of meso-dibromobutane, the major compound formed is (a) \(\mathrm{n}\)-butane (b) 1 -butane (c) cis-2-butene (d) trans-2-butene

Short answer

The major compound formed is trans-2-butene (d).

Step-by-step solution

Identify the Reaction Type

Debromination is the process of removing bromine atoms from a compound. In the case of meso-dibromobutane, this process will involve an elimination reaction, specifically an E2 dehalogenation reaction, where the extit{meso} form will lose the two bromine atoms to form an alkene.

Determine the Stereochemistry

meso-dibromobutane has one bromine atom on each of two adjacent carbon atoms, with a configuration that allows for symmetry. When considering how the two bromine atoms are eliminated, it's important to consider the anti-periplanar geometry required for an E2 reaction, which will influence the double bond configuration in the resulting alkenes.

Predict the Alkene Configuration

During the E2 reaction, the two hydrogen atoms in the antiperiplanar position to the bromine atoms are eliminated along with the bromines. This elimination forms a double bond between the two central carbon atoms, favoring the formation of trans-2-butene because the trans (E) configuration is more stable due to less steric strain compared to the cis (Z) configuration.

Conclusion Based on Stability

Trans-2-butene is the more stable product due to lower steric hindrance compared to cis-2-butene. The stability often determines the major product in such reactions, making trans-2-butene the major compound formed.

Key concepts

E2 Elimination Reaction

E2 elimination reactions are a type of reaction where a molecule loses elements to form a double bond. This reaction involves the simultaneous removal of a proton (hydrogen) and a leaving group (bromine, in this case), resulting in the formation of an alkene. Here, the process is termed 'bimolecular' because the rate of reaction depends on the concentration of two reactants. To form the double bond efficiently, the leaving group and the hydrogen atom being removed should ideally be in an 'anti-periplanar' position. This means they are on opposite sides of the molecule, which helps in the smooth breaking and forming of bonds. In the debromination of meso-dibromobutane, this elimination leads primarily to the formation of an alkene, specifically 2-butene.

Stereochemistry

Stereochemistry focuses on the spatial arrangement of atoms in molecules and their manipulation during chemical reactions. In E2 reactions, stereochemistry plays a crucial role because the geometry of the molecule determines which hydrogen atom can be removed. For the E2 mechanism to occur, the hydrogen and the bromine must be positioned in an anti-periplanar configuration. This means they should be directly aligned but on opposite sides of the rotating carbon-carbon axis. In our case with meso-dibromobutane, this stereochemical requirement dictates how bromine atoms are removed and heavily influences the final product's shape.

Alkene Configuration

Once the E2 elimination reaction occurs, the resulting alkene's configuration becomes a key focus. Alkenes can have different geometric configurations, mainly referred to as 'cis' and 'trans', or more formally, 'Z' (Zusammen - together) for cis and 'E' (Entgegen - opposite) for trans. In meso-dibromobutane's debromination, the anti-periplanar arrangement facilitates the formation of the trans (E) isomer of 2-butene. This configuration is generally more favored because the substituents on the carbon-carbon double bond are farther apart, reducing steric hindrance. Thus, debromination primarily produces trans-2-butene as the main product.

Stability of Alkenes

The stability of alkenes is greatly influenced by their configuration. Generally, trans isomers (E) are more stable than their cis (Z) counterparts. This is because the trans configuration minimizes repulsive forces between bulky groups, leading to reduced steric hindrance. For 2-butene, trans-2-butene is more stable than cis-2-butene for this reason. In chemical reactions like the debromination of meso-dibromobutane, the more stable alkene product is typically the major product, which is why trans-2-butene predominates. The understanding of alkene stability is crucial in predicting the major products in elimination reactions.