Question

question 9 (10 points) listen what is the structure of 2-methyl-3-hexene?
ch₃−ch₂
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c=c ch₃
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h ch₂−ch₃

ch₃ h
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c=c
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ch₃ ch₂−ch₂−ch₃

(ch₃)₂c=chch₂ch₂ch₃

(ch₃)₂chch=chch₂ch₃

none of the answers relate to the structure of 2-methyl-3-hexene.

Explanation:

To determine the structure of 2 - methyl - 3 - hexene, we first analyze the name:

Step 1: Identify the parent chain and double bond position

- The parent chain is hexene, so it has 6 carbon atoms with a double bond at the 3rd carbon (from the numbering).
- There is a methyl group ($-CH_3$) at the 2nd carbon.

Step 2: Analyze each option

- **First option**: Let's expand the structure. The carbon chain: The double bond is between carbons (let's number them). Let's number the carbons in the first structure. The left - most carbon is $CH_3 - CH_2 - C$(with $H$ and double bond to next $C$) $=C$(with $CH_3$ and $CH_2 - CH_3$). Numbering the carbons: C1: $CH_3$, C2: $CH_2$, C3: $C$ (with $H$), C4: $C$ (with $CH_3$ and $CH_2 - CH_3$), C5: $CH_2$, C6: $CH_3$. Wait, no, the parent chain here: Let's re - number. The double bond is between C3 and C4? Wait, no, let's do proper IUPAC numbering. For the first structure: The carbon atoms in the chain: Let's list them as C1: $CH_3$, C2: $CH_2$, C3: $C$ (attached to $H$), C4: $C$ (attached to $CH_3$ and $CH_2 - CH_3$), C5: $CH_2$, C6: $CH_3$. The double bond is between C3 and C4. The methyl group: Is there a methyl at C2? No, in this structure, the substituents: C2 is $CH_2$, C4 has $CH_3$. So this is not 2 - methyl - 3 - hexene.
- **Second option**: The structure is $CH_3 - C(CH_3)=C(H)-CH_2 - CH_2 - CH_3$. Numbering the carbons: C1: $CH_3$, C2: $C(CH_3)$, C3: $C(H)$, C4: $CH_2$, C5: $CH_2$, C6: $CH_3$. The double bond is between C2 and C3. But we need the double bond at C3, so this is incorrect.
- **Third option**: $(CH_3)_2C = CHCH_2CH_2CH_3$. Let's number the carbons. C1: $(CH_3)_2C$, C2: $CH$, C3: $CH_2$, C4: $CH_2$, C5: $CH_3$? Wait, no, the formula is $(CH_3)_2C=CHCH_2CH_2CH_3$. Let's count the carbon atoms: The first carbon is $(CH_3)_2C$ (so C1), then C2: $CH$, C3: $CH_2$, C4: $CH_2$, C5: $CH_3$? Wait, no, the parent chain should be 6 carbons. Wait, $(CH_3)_2C$ is C1, then $CH$ (C2), $CH_2$ (C3), $CH_2$ (C4), $CH_3$ (C5)? No, that's 5 carbons in the chain? Wait, no, $(CH_3)_2C=CHCH_2CH_2CH_3$: Let's count the carbon atoms. The first part: $(CH_3)_2C$ (1 C), then $CH$ (2nd C), $CH_2$ (3rd C), $CH_2$ (4th C), $CH_3$ (5th C)? No, that's wrong. Wait, maybe I miscounted. Wait, $(CH_3)_2C$ is one carbon with two methyl groups, then $CH$ (second carbon), $CH_2$ (third), $CH_2$ (fourth), $CH_3$ (fifth)? No, that's 5 carbons. But hexene has 6 carbons. So this is incorrect.
- **Fourth option**: $(CH_3)_2CHCH = CHCH_2CH_3$. Let's number the carbons. C1: $(CH_3)_2CH$ (so C1 has two methyl groups and a $H$), C2: $CH$, C3: $CH = CH$? Wait, no, let's write the structure out. $(CH_3)_2CH - CH = CH - CH_2 - CH_3$. Now number the carbons: C1: $(CH_3)_2CH$ (C1), C2: $CH$, C3: $CH$, C4: $CH$, C5: $CH_2$, C6: $CH_3$. The double bond is between C3 and C4 (since $CH = CH$ is between C3 and C4). The methyl groups: At C2? Wait, no, $(CH_3)_2CH$ is C1, so C2 is $CH$, C3 is $CH$, C4 is $CH$, C5 is $CH_2$, C6 is $CH_3$. Wait, no, let's do proper IUPAC numbering. We number the chain to give the double bond the lowest number. The double bond is between C3 and C4. Now, the substituent: a methyl group at C2? Wait, in $(CH_3)_2CHCH = CHCH_2CH_3$, the C2 is $CH$ (attached to C1: $(CH_3)_2CH$ and C3: $CH$). Wait, no, C1: $(CH_3)_2CH$ (so C1 has two $CH_3$ and one $H$), C2: $CH$ (attached to C1 and C3), C3: $CH$ (attached to C2 and C4), C4: $CH$ (attached to C3 and C5), C5: $CH_2$ (attached to C4 and C6), C6: $CH_3$. Wait, no, the formula is $(CH_3)_2CHCH = CHCH_2CH_3$. Let's count the carbon atoms: C1: $(CH_3)_2CH$ (1 C), C2: $CH$ (2nd C), C3: $CH$ (3rd C), C4: $CH$ (4th C), C5: $CH_2$ (5th C), C6: $CH_3$ (6th C). The double bond is between C3 and C4. Now, the substituent: Is there a methyl at C2? Wait, C2 is $CH$ (attached to C1: $(CH_3)_2CH$). Wait, no, C1 is $(CH_3)_2CH$, so C1 has two $CH_3$ groups. So the structure is: C1: $CH_3$, $CH_3$, $H$ (attached to C2), C2: $CH$ (attached to C1, C3), C3: $CH$ (attached to C2, C4), C4: $CH$ (attached to C3, C5), C5: $CH_2$ (attached to C4, C6), C6: $CH_3$. Wait, no, the correct way: Let's draw the structure. The parent chain is 6 carbons: $C - C - C = C - C - C$. Now, at C2, we have a methyl group ($-CH_3$), so C2: $CH_3$ (and the other bonds: to C1 and C3). Wait, no, the name is 2 - methyl - 3 - hexene. So the parent chain is hexene (6 C), double bond at C3 - C4, methyl at C2. So the structure should be: C1: $CH_3$, C2: $CH_3$ (attached to C1 and C3), C3: $CH$ (attached to C2 and C4), C4: $CH$ (attached to C3 and C5), C5: $CH_2$ (attached to C4 and C6), C6: $CH_3$. Wait, no, let's write the correct formula. The correct structure for 2 - methyl - 3 - hexene is: $CH_3 - CH(CH_3)-CH = CH - CH_2 - CH_3$, which is the same as $(CH_3)_2CHCH = CHCH_2CH_3$. Let's check the numbering: The double bond is between C3 and C4 (since we number from the end closest to the double bond). The methyl group is at C2 (since $CH(CH_3)$ is at C2).

Now let's re - check the options:

- First option: The structure is $CH_3 - CH_2 - C(H)=C(CH_3)-CH_2 - CH_3$. Let's number the carbons: C1: $CH_3$, C2: $CH_2$, C3: $C(H)$, C4: $C(CH_3)$, C5: $CH_2$, C6: $CH_3$. The double bond is between C3 and C4. The methyl group is at C4, not C2. So this is 4 - methyl - 3 - hexene, not 2 - methyl - 3 - hexene.
- Second option: $CH_3 - C(CH_3)=C(H)-CH_2 - CH_2 - CH_3$. Numbering: C1: $CH_3$, C2: $C(CH_3)$, C3: $C(H)$, C4: $CH_2$, C5: $CH_2$, C6: $CH_3$. Double bond at C2 - C3, methyl at C2. So this is 2 - methyl - 2 - hexene, not 2 - methyl - 3 - hexene.
- Third option: $(CH_3)_2C = CHCH_2CH_2CH_3$. Numbering: C1: $(CH_3)_2C$, C2: $CH$, C3: $CH_2$, C4: $CH_2$, C5: $CH_3$? No, wait, the formula is $(CH_3)_2C=CHCH_2CH_2CH_3$. Let's count the carbons: C1: $(CH_3)_2C$ (1 C), C2: $CH$ (2nd C), C3: $CH_2$ (3rd C), C4: $CH_2$ (4th C), C5: $CH_3$ (5th C)? No, that's 5 carbons in the chain? Wait, no, $(CH_3)_2C$ is one carbon, then $CH$ (2nd), $CH_2$ (3rd), $CH_2$ (4th), $CH_3$ (5th)? No, that's 5 carbons. But hexene has 6 carbons. So this is incorrect.
- Fourth option: $(CH_3)_2CHCH = CHCH_2CH_3$. Let's number the carbons: C1: $(CH_3)_2CH$ (C1), C2: $CH$ (C2), C3: $CH$ (C3), C4: $CH$ (C4), C5: $CH_2$ (C5), C6: $CH_3$ (C6). The double bond is between C3 and C4. The methyl groups are at C1? Wait, no, $(CH_3)_2CH$ is C1, so C1 has two $CH_3$ groups. Wait, no, the correct numbering: We should number the chain so that the double bond has the lowest possible number. Let's start numbering from the right - hand side. C6: $CH_3$, C5: $CH_2$, C4: $CH$, C3: $CH$, C2: $CH$, C1: $(CH_3)_2CH$. No, that's not right. Wait, let's write the structure as a straight chain. The correct structure for 2 - methyl - 3 - hexene is:

Carbon 1: $CH_3$

Carbon 2: $CH(CH_3)$ (so a methyl group at C2)

Carbon 3: $CH$

Carbon 4: $CH$

Carbon 5: $CH_2$

Carbon 6: $CH_3$

And the double bond is between C3 and C4: $CH_3 - CH(CH_3)-CH = CH - CH_2 - CH_3$, which is the same as $(CH_3)_2CHCH = CHCH_2CH_3$.

Answer:

\(\boldsymbol{(CH_3)_2CHCH = CHCH_2CH_3}\) (the fourth option)