Gaokao Exam Chemistry: Organic Chemistry Fundamentals 2 — Questions and Answers

Question 1: Which of the following is the correct IUPAC name for CH₃-CH(OH)-CH₂CH₃?

2-butanol (Correct answer)
3-butanol
1-methylpropanol
2-methylpropan-1-ol

Correct answer: 2-butanol

The longest chain has 4 carbons (butane). The -OH group is on carbon 2, making it 2-butanol.

The compound CH₃-CH(OH)-CH₂CH₃ has a four-carbon chain. Numbering from the end closest to the hydroxyl group places -OH on carbon 2. The parent chain is butane, so the compound is named 2-butanol. This is a secondary alcohol because the -OH-bearing carbon is attached to two other carbons.

Question 2: Benzene reacts with bromine in the presence of FeBr₃ catalyst. What type of reaction occurs?

Electrophilic substitution (Correct answer)
Addition reaction
Free radical substitution
Nucleophilic substitution

Correct answer: Electrophilic substitution

Benzene undergoes electrophilic aromatic substitution (EAS) with Br₂/FeBr₃. FeBr₃ activates Br₂ to form the electrophile Br⁺.

Benzene's delocalized π electron system makes it an electron-rich substrate that attracts electrophiles. In the presence of Lewis acid FeBr₃, bromine is activated to form the electrophilic Br⁺ species. The benzene ring attacks this electrophile in an electrophilic aromatic substitution (EAS) reaction, regenerating aromaticity by losing H⁺ rather than adding across the double bond.

Question 3: What is the degree of unsaturation (DBE) of the molecular formula C₆H₆?

4 (Correct answer)
3
2
6

Correct answer: 4

DBE = (2×6 + 2 - 6)/2 = (12 + 2 - 6)/2 = 8/2 = 4. Benzene has 3 double bonds + 1 ring = 4 DBE.

The degree of unsaturation (DBE) formula for CₙHₘ is DBE = (2n + 2 - m)/2. For C₆H₆: DBE = (2×6 + 2 - 6)/2 = 8/2 = 4. This reflects benzene's structure: three C=C double bonds (3 DBE) plus one ring (1 DBE) = 4 total degrees of unsaturation.

Question 4: Ethanol undergoes oxidation with acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄). What is the primary product under controlled conditions?

Ethanal (acetaldehyde) (Correct answer)
Ethanoic acid (acetic acid)
Ethene
Diethyl ether

Correct answer: Ethanal (acetaldehyde)

Under controlled (mild) oxidation of a primary alcohol, the first product is an aldehyde (ethanal). Further oxidation gives the carboxylic acid.

Ethanol (CH₃CH₂OH) is a primary alcohol. With K₂Cr₂O₇/H₂SO₄ under controlled conditions (e.g., using excess alcohol and distilling off the product), it is oxidized to ethanal (CH₃CHO, acetaldehyde). Under more vigorous or excess oxidant conditions, further oxidation occurs to give ethanoic acid (CH₃COOH, acetic acid).

Question 5: Which functional group is present in an ester?

-COO- (Correct answer)
-CHO
-COOH
-OH

Correct answer: -COO-

Esters contain the -COO- (ester linkage) functional group, formed from a carboxylic acid and an alcohol by condensation.

Esters have the general formula R-COO-R', containing the ester group -COO-. They are formed by the condensation reaction between a carboxylic acid (-COOH) and an alcohol (-OH) with loss of water. The ester linkage consists of a carbonyl group (C=O) bonded to an oxygen atom (-O-), giving the distinctive -COO- group.

Question 6: Which statement about alkanes is correct according to the Chinese high school chemistry curriculum?

They undergo substitution reactions with halogens under UV light (Correct answer)
They readily undergo addition reactions
They are soluble in water
They react with sodium hydroxide solution

Correct answer: They undergo substitution reactions with halogens under UV light

Alkanes are relatively unreactive but do undergo free radical substitution with halogens (e.g., Cl₂) under UV light or heat.

Alkanes are saturated hydrocarbons with only C-C and C-H single bonds. Their primary chemical reaction is halogenation via free radical substitution, which requires UV light or high temperature to initiate the chain reaction (homolytic cleavage of Cl₂). Alkanes do not undergo addition reactions (those require double/triple bonds), are insoluble in water, and do not react with NaOH.

Gaokao Exam Chemistry: Organic Chemistry Fundamentals 2

Which of the following is the correct IUPAC name for CH₃-CH(OH)-CH₂CH₃?