Question

The IUPAC name of the compound is (a) 6-bromo-2-chlorocyclohexene (b) 3-bromo-1-chlorocyclohexene (c) 1 -bromo-3-chlorocyclohexene (d) 2-bromo-6-chlorocyclohexene

Short answer

a) 6-bromo-2-chlorocyclohexene b) 3-bromo-1-chlorocyclohexene c) 1-bromo-3-chlorocyclohexene d) 2-bromo-6-chlorocyclohexene Answer: c) 1-bromo-3-chlorocyclohexene

Step-by-step solution

Identify the parent chain

In this case, the parent chain is cyclohexene, which is a 6-membered ring with one double bond. All of the options have this as their base name, so we can continue to the next step.

Assign numbering to the substituents

We have two substituents: bromine (Br) and chlorine (Cl). According to IUPAC naming rules, the lower locant number should be assigned to the substituent nearest to the double bond of cyclohexene. If there is a tie, then the alphabetical order should break that tie. We analyze each option: (a) 6-bromo-2-chlorocyclohexene: Here, bromine is at the 6th position, and chlorine is at the 2nd position. (b) 3-bromo-1-chlorocyclohexene: Bromine is at the 3rd position and chlorine is at the 1st position. In this case, the chlorine is closest to the double bond, which is preferable according to the naming rules. (c) 1-bromo-3-chlorocyclohexene: Bromine is at the 1st position, and chlorine is at the 3rd position. Here, bromine is closest to the double bond. (d) 2-bromo-6-chlorocyclohexene: Bromine is at the 2nd position, and chlorine is at the 6th position.

Choose the correct option based on numbering and alphabetical order

After analyzing the numbers assigned to the substituents in each option and checking the alphabetical order rule, the correct choice should be (c) 1-bromo-3-chlorocyclohexene. In this option, the bromine is closest to the double bond and it is also consistent with alphabetical order rules. So, the IUPAC name of the given compound is 1-bromo-3-chlorocyclohexene.

Key concepts

Organic Chemistry

Organic chemistry is the study of carbon-containing compounds, which are central to life on Earth. This branch of chemistry explores the structure, properties, reactions, and preparation of carbon-based substances. These compounds can include elements like hydrogen, oxygen, nitrogen, and halogens such as chlorine and bromine.
The discipline is vital for the development of pharmaceuticals, plastics, petrochemicals, food, and much more. Students often learn about various chemical reactions and how they form the basis for synthesizing new compounds.

• Hydrocarbons: These are compounds composed entirely of carbon and hydrogen. They can be aliphatic (chains) or aromatic (rings).
• Functional Groups: Specific groupings of atoms within molecules that have distinctive chemical properties.
• Nomenclature: Systematic naming of organic compounds, ensuring each chemical entity has a unique name for scientific communication.

Cyclohexene

Cyclohexene is an organic compound and an example of a cycloalkene. It is composed of a six-membered ring with one double bond, making it a cyclic structure. The presence of the double bond introduces unsaturation, which affects how this compound reacts chemically.
Due to the double bond, cyclohexene has specific reactivity patterns, making it susceptible to reactions such as addition, oxidation, and polymerization. Its study includes understanding this behavior and applying it to synthesize other useful compounds.

• Structure: A six-carbon ring with one double bond.
• Properties: Colorless liquid at room temperature, with a faint odor.
• Reactions: Reacts with hydrogen, halogens, and other compounds, often involving the double bond.

Substituent Numbering

Substituent numbering is a crucial part of naming organic compounds. It helps in identifying the exact positions of functional groups or substituents around a ring or chain.
In cyclohexene, when numbering, the priority is to assign the lowest set of numbers to the substituents, especially near the double bond, as it is considered the point of unsaturation. If substituents are equidistant from the reference point, the alphabetical order is used to resolve the numbering.

• Priority Rules: The lowest locant rule takes precedence, reducing ambiguity in the naming.
• Double Bond Influence: Numbering starts nearest to the double bond.
• Resolving Ties: When equal distances occur, alphabetical precedence decides numbering.

Alphabetical Order in Naming

Alphabetical order plays a significant role in the IUPAC nomenclature, particularly when choosing between numbering schemes that result in a tie. This ensures consistency and predictability in naming conventions.
When assigning numbers, the order of appearance of substituent names in the alphabet determines priority if two possible substitutions are equidistant from the reference point. Substituents are named by their specific names and not their prefixes like di-, tri-, etc.

• Tie Situations: Use alphabetical order to determine which substituent receives the lower number.
• Exclusions: Ignore prefixes that denote quantity or position when determining alphabetical order.
• Standardization: Ensures that all chemists understand the structure from its name without ambiguity.