What is the Difference Between Geminal and Vicinal Dihalides

Dihalides are organic molecules with two halogen atoms bonded to a carbon chain. Dihalides can be formed by adding halogens to alkenes (double-bonded carbon chains). Due to the arrangement of halides in the molecule, dihalides can be classified as vicinal dihalides and geminal dihalides.

What is the difference between geminal and vicinal dihalides? In geminal dihalide, both halogen atoms are attached to the same carbon atom, whereas in vicinal dihalide, two halogen atoms are attached to adjacent carbon atoms.

Key Areas Covered

1. What is Geminal Dihalide
      – Definition, Features 
2. What is Vicinal Dihalide 
      – Definition, Features
3. Similarities Between Geminal and Vicinal Dihalides
      – Outline of Common Features
4. Difference Between Geminal and Vicinal Dihalides
      – Comparison of Key Differences
5. FAQ: Geminal and Vicinal Dihalides
      – Answers to Frequently Asked Questions

Key Terms

Geminal Dihalides, Vicinal Dihalides

What is Geminal Dihalide

Geminal dihalides, also known as alkylidene halides, are a type of organic molecule with a specific arrangement of halogen atoms. The main feature of these molecules is that two identical halogen atoms are attached to the same carbon atom within the hydrocarbon chain. For example, chlorine (Cl) or bromine (Br) become partners and bond with the same carbon atom on the chain.

Examples of geminal dihalides include 1,1-dichloroethane (CH₃CHCl₂), where two chlorine atoms are attached to the second carbon atom (indicated by the “1,1”). Another example is 2,2-dibromopropane (CH₃CHBr₂), where both bromine atoms reside on the central carbon (position “2”). One main reaction involving geminal dihalides is geminal dihalide hydrolysis. Here, the molecule reacts with water to form carbonyl compounds.

Figure 1: 1,1-dichloroethane

Depending on the position of the geminal dihalides on the carbon chain, the products can be either aldehydes (from primary geminal dihalides) or ketones (from secondary geminal dihalides). Though there are not many uses of geminal dihalides in the industry due to its reactivity, they can act as precursors for carbonyl compounds and can act as organic synthesis intermediates.

What are Vicinal Dihalides

Vicinal dihalides are organic molecules with a specific arrangement of halogen atoms. They contain two halogen atoms attached to adjacent carbon atoms within the hydrocarbon chain. An example is 1,2-dichloroethane (ClCH₂CH₂Cl). Here, the chlorine atoms reside on the first and second carbon atoms (indicated by “1,2”). Another example is 1,1,2,2-tetrabromoethane (Br₂CH₂CH₂Br₂), where two bromine pairs occupy adjacent carbon positions (1 and 2).

Figure 2: 1,1,2,2-tetrabromoethane

Vicinal dihalides are involved in organic synthesis due to their reactivity. Moreover, one of the main reactions that involve vinal dihalides is their conversion to alkenes (compounds with double carbon bonds).

Vicinal dihalides undergo dehydrohalogenation when they react with strong bases such as alcoholic KOH. This process eliminates a molecule of hydrogen halide (like HCl) and creates a double bond between the carbons that originally held the halogens. They also undergo nucleophilic substitution reactions depending on the specific functional groups and the halogen present.

Similarities Between Geminal and Vicinal Dihalides

1. Both are classified as dihalides, meaning they contain two halogen atoms covalently bonded to the carbon chain.
2. Moreover, they serve as starting materials for the synthesis of organic compounds.
3. Both types can undergo reactions with strong bases like alcoholic KOH.
4. In addition, both can be formed through addition reactions of alkenes with halogens.

Difference Between Geminal and Vicinal Dihalides

Definition

• Geminal dihalides are organic compounds where two halogen atoms are attached to the same carbon atom, whereas vicinal dihalides are organic compounds where two halogen atoms are attached to adjacent carbon atoms.

Hydrolysis

• Geminal dihalide hydrolysis (reaction with water) yields aldehydes or ketones depending on the original carbon position. However, vicinal dihalide hydrolysis doesn’t typically occur due to the lack of a cleavable bond between the carbons.

Formation

• Geminal dihalides undergo hydrolysis to form aldehydes/ketones, while vicinal dihalides undergo dehydrohalogenation to form alkenes.

Addition

• Geminal dihalides often involve a syn addition (both halogens add to the same side), while vicinal dihalides typically involve an anti-addition (halogens add to opposite sides).

Conclusion

In organic chemistry, there are two types of dihalides: vicinal dihalides and geminal dihalides. In geminal dihalides, the two halogen atoms are bonded to the same carbon atom. However, vicinal dihalides have two halogen atoms bonded to adjacent carbon atoms. This is the main difference between geminal and vicinal dihalides.

FAQ: Geminal and Vicinal Dihalides

1. What are vicinal and geminal diols?

Vicinal and geminal diols are both molecules with two hydroxyl (OH) groups. Vicinal diols have the OH groups on neighboring carbons, while geminal diols have both OH groups bonded to the same carbon, which is unstable.

2. What are the different types of Dihalides?

There are two main types of dihalides depending on the relative position of the two halogen atoms on a carbon chain. In geminal dihalides, the two halogen atoms are bonded to the same carbon atom. Vicinal dihalides have the two halogen atoms bonded to adjacent carbon atoms.

3. How to make geminal dihalide?

Synthesis of geminal dihalides can involve:

• Adding a halogen (like chlorine) to an alkyne (carbon triple bond).
• Reacting a non-enolizable aldehyde or ketone with specific reagents like phosgene.

4. Are vicinal dihalides colorless?

Yes, most vicinal dihalides are colorless liquids or solids. The color of a molecule is influenced by electron transitions, and vicinal dihalides generally lack the structural features to absorb visible light.

5. How are vicinal dihalides formed from alkenes?

Vicinal dihalides form from alkenes through addition reactions with halogens (like chlorine or bromine). This typically occurs in a double bond, resulting in two halogen atoms on adjacent carbons.

Reference:

1. “Geminal Dihalide.” Encyclopedia Britannica.
2. “Vicinal Dihalide.” Encyclopedia Britannica.

Image Courtesy:

1. “1,1-Dichloroethane” By The original uploader was Sbrools at English Wikipedia. – Transferred from en.wikipedia to Commons. (CC BY-SA 3.0) via Commons Wikimedia
2. “Tetrabromoethane” By Hbf878 – Own work (Public Domain) via Commons Wikimedia