Problem 98 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

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

Step 1: Understanding Debromination

Debromination is a chemical reaction that involves the removal of bromine atoms from a molecule. In the case of meso-dibromobutane, which is a type of dibromo-alkane, this will result in the formation of a double bond as each bromine atom is eliminated, producing an alkene.

Step 2: Analyze the Structure of meso-Dibromobutane

The meso form of dibromobutane has bromine atoms on the second and third carbons in opposite configurations (one on the wedge and one on the dash, if represented in a 3D model). This specific arrangement is crucial for determining which isomer of butene will be predominant after debromination as it dictates the stereochemistry of the potential product.

Step 3: Identify Possible Products

Once the bromine atoms are removed, the most stable alkene is formed. Considering the position and stereochemistry of bromine in meso-dibromobutane, the potential products are cis-2-butene and trans-2-butene. The higher stability of alkene structures is often found in trans configurations due to reduced steric hindrance between substituent groups on the double bond.

Step 4: Determine the Major Product

In elimination reactions like debromination, the trans isomer is typically more stable than the cis isomer due to lower steric hindrance. Therefore, for meso-dibromobutane, debromination results in the formation of trans-2-butene as the major product.

Key Concepts

Meso-Dibromobutane

Meso-dibromobutane is an interesting compound in the realm of organic chemistry. It is an example of a dibromo-alkane where two bromine (Br) atoms are attached at the second and third carbon atoms of the butane chain. This spatial arrangement is known as meso because it exhibits an internal plane of symmetry. Despite having chiral centers, the overall molecule is achiral due to this symmetry.

Understanding this structural feature is key when discussing chemical reactions like debromination, as the symmetrical arrangement influences the stereochemistry of the resulting products. The symmetry ensures that the stereo isomers will adopt a certain configuration upon removal of the bromines. In this case, it leads to the formation of alkenes with particular isomeric forms, such as trans-2-butene.

Stereochemistry

Stereochemistry refers to the 3D arrangement of atoms within a molecule and is fundamental in determining the properties and reactions of chemical compounds. For meso-dibromobutane, understanding its stereochemistry is crucial, especially during debromination.

As the bromine atoms are positioned in opposite configurations (one on the wedge, one on the dash), this 3D arrangement affects which isomer will form. This happens because the stereochemical arrangement will dictate how the atoms in the molecule will spatially reconfigure as they eliminate bromine pairs and form a double bond. The concept of stereochemistry also helps us predict reaction outcomes, such as the formation of a more stable trans-2-butene rather than the cis isomer.

Cis-Trans Isomerism

Cis-trans isomerism, also known as geometric isomerism, arises in compounds that have restricted rotation around a bond, usually a double bond. When meso-dibromobutane undergoes debromination, a double bond forms between the second and third carbon atoms, paving the way for cis-trans isomerism.

The cis isomer has both substituent groups on the same side, leading to increased steric hindrance due to proximity. Meanwhile, the trans isomer has groups on opposite sides, reducing steric repulsion and often resulting in a more stable molecule. In the case of meso-dibromobutane, debromination predominantly yields the trans isomer, namely trans-2-butene, because of its lower steric hindrance compared to the cis form.

Alkene Stability

Alkene stability is a crucial concept when predicting the outcomes of debromination reactions. When the bromine atoms are eliminated from meso-dibromobutane, an alkene with a carbon-carbon double bond is formed. The stability of this new alkene is an important consideration and often determines the major product of the reaction.

The transisomer of an alkene is generally more stable than the cis isomer. This is due to less steric strain between substituents when positioned across from each other. As a result, during debromination of meso-dibromobutane, trans-2-butene becomes the predominant product. This tendency for trans isomer preference aligns with the general observation that more substituted and symmetrical alkenes tend to be more stable, due to reduced steric interactions.