Question

determine the iupac name of the below organic compound.

a. 5 – bromo – 3 methyl – pent – 3 – ene

b. 1 – bromo – 3 methyl – pent – 2 – ene

c. 1 – bromo – 3 ethyl – but – 2 – ene

d. 3 – bromo – 4 methyl – pent – 2 – ene

e. 1 – bromo -- pent – 2 – ene

Explanation:

Step1: Identify the parent chain

The parent chain should be the longest carbon chain containing the double bond. Let's count the carbons: the chain with the double bond and the bromine - let's see the structure. The double bond is between carbons, and we need to number the chain to give the double bond the lowest possible number, and also consider the substituents (bromine and methyl). Let's draw the structure mentally: the carbon chain has 5 carbons (pent - ), the double bond is at position 2 (since we number from the end that gives the double bond the lowest number, and also the bromine and methyl positions). Wait, let's number the carbons properly. Let's start numbering from the end closer to the double bond and the substituents. Let's see the options:

First, the parent chain length: let's check each option. Option c has but - (4 carbons), but our chain is 5 carbons (pent - ), so c is out. Option e has no methyl, but our structure has a methyl, so e is out. Now, let's number the carbons for the remaining options (a, b, d).

For the correct numbering: the double bond is at position 2 (since we want the double bond to have the lowest number). Now, the bromine is at position 1? Wait, no, let's look at the structure. The structure: let's parse the given structure. The carbon with the double bond: let's see, the chain is: carbon 1 (with Br), carbon 2, carbon 3 (double bond between carbon 2 and 3? Wait, no, the structure: the double bond is between two carbons, and there is a methyl on one of the carbons, and a bromine on the end. Wait, let's draw the structure:

Carbon chain: C - C(CH3)=C - C - Br? No, wait the given structure: the left part has a ethyl? No, wait the structure: the first carbon (left) is connected to a chain, then a carbon with a methyl (CH3) and a double bond, then a carbon, then a carbon with Br. Wait, let's count the carbons in the parent chain. Let's see: the longest chain with the double bond: let's count the carbons. Let's number from the end with Br? No, numbering should give the double bond the lowest possible number, and then the substituents (bromine and methyl) the lowest possible numbers.

Let's assign numbers:

Start numbering from the end closer to the double bond. Let's see the double bond is between carbon 2 and 3? Wait, no, let's look at the options. Option b: 1 - Bromo - 3 methyl - pent - 2 - ene. Let's check the parent chain: pent (5 carbons). Double bond at 2, bromine at 1, methyl at 3. Let's see the structure: carbon 1: Br, carbon 2: double bond with carbon 3, carbon 3: methyl, carbon 4, carbon 5? Wait, no, pent is 5 carbons. So carbon 1: Br, carbon 2: =, carbon 3: CH3, carbon 4, carbon 5? Wait, no, the chain is C1(Br)-C2=C3(CH3)-C4-C5? No, that's 5 carbons. Wait, but the double bond is between C2 and C3, bromine at C1, methyl at C3. Let's check the numbering: the double bond is at position 2 (lowest possible), bromine at 1, methyl at 3. Now check other options:

Option a: 5 - Bromo - 3 methyl - pent - 3 - ene. Double bond at 3, which is higher than 2, so not good.

Option d: 3 - Bromo - 4 methyl - pent - 2 - ene. Bromine at 3, which is higher than 1, so not good.

Option c: but - (4 carbons), wrong length.

Option e: no methyl, wrong.

So the correct numbering is: parent chain pentane (5 carbons), double bond at 2, bromine at 1, methyl at 3. So the name is 1 - Bromo - 3 methyl - pent - 2 - ene.

Step2: Verify the numbering rules

IUPAC naming rules: number the parent chain to give the double bond the lowest possible number, then number substituents (bromine and methyl) with the lowest possible numbers. The parent chain is 5 carbons (pent), double bond at 2, bromine at 1, methyl at 3. So the name is 1 - Bromo - 3 methyl - pent - 2 - ene, which is option b.

Answer:

b. 1 - Bromo - 3 methyl - pent - 2 - ene