IB Chemistry: Identification and Properties of Common Functional Groups | IB化学：常见官能团的识别与性质

📚 IB Chemistry: Identification and Common Functional Groups | IB化学：常见官能团的识别与性质

A functional group is a specific atom or group of atoms within a molecule that is responsible for its characteristic chemical reactions. In IB Chemistry, recognising functional groups is essential for predicting reactivity, naming organic compounds, and interpreting reaction pathways.

官能团是分子中决定其特征化学反应的特定原子或原子团。在IB化学中，识别官能团对于预测反应性、命名有机化合物和理解反应路线至关重要。

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1. The Concept of Functional Groups | 官能团的概念

Organic compounds are organised into homologous series, where each member contains the same functional group and differs by a CH₂ unit. The functional group determines the physical and chemical behaviour of the molecule, while the carbon skeleton mainly influences solubility, boiling point, and intermolecular forces.

有机化合物按同系列排列，同一系列中每个成员含有相同的官能团，并且相差一个CH₂单元。官能团决定分子的物理和化学行为，而碳骨架主要影响溶解度、沸点和分子间作用力。

• Functional groups are shown in structural formulas, e.g. C=C, -OH, -COOH, -NH₂, -C≡N.

  官能团在结构式中显示，例如C=C、-OH、-COOH、-NH₂、-C≡N。

• Multiple functional groups can exist in one molecule, and their priority determines the IUPAC suffix.

  一个分子中可存在多个官能团，其优先顺序决定IUPAC命名中的后缀。

• Reactions usually occur at the functional group, leaving the hydrocarbon skeleton unchanged.

  反应通常发生在官能团上，而烃骨架保持不变。

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2. Alkenes: The C=C Group | 烯烃：碳碳双键

Alkenes contain a carbon-carbon double bond consisting of one sigma (σ) bond and one pi (π) bond. The π bond is electron-rich and makes alkenes reactive towards electrophiles.

烯烃含有碳碳双键，由一个sigma（σ）键和一个pi（π）键组成。π键电子密度高，使烯烃容易与亲电试剂反应。

Identification: Alkenes decolourise bromine water (orange to colourless) and acidified potassium manganate(VII) (purple to colourless).

识别：烯烃能使溴水褪色（橙色变为无色），也能使酸化高锰酸钾溶液褪色（紫色变为无色）。

Key reactions:

主要反应：

• Electrophilic addition of HBr or H₂O: CH₂=CH₂ + HBr → CH₃CH₂Br.

  HBr或H₂O的亲电加成：CH₂=CH₂ + HBr → CH₃CH₂Br。

• Addition polymerisation: many alkene monomers form polyalkenes, e.g. poly(ethene).

  加成聚合：许多烯烃单体形成聚烯烃，例如聚乙烯。

• Hydrogenation: alkene + H₂ → alkane, using a Ni or Pt catalyst.

  加氢反应：烯烃 + H₂ → 烷烃，使用Ni或Pt催化剂。

C₂H₄ + Br₂ → C₂H₄Br₂

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3. Alcohols: The -OH Group | 醇：羟基

Alcohols have a hydroxyl group (-OH) attached to a saturated carbon atom. They are classified as primary, secondary, or tertiary depending on how many carbon atoms are bonded to the carbon bearing the -OH group.

醇含有与饱和碳原子相连的羟基（-OH）。根据连接羟基的碳原子所键连的碳原子数目，醇可分为伯醇、仲醇和叔醇。

• Primary alcohol: RCH₂OH — oxidised to aldehyde, then carboxylic acid.

  伯醇：RCH₂OH — 氧化生成醛，再氧化生成羧酸。

• Secondary alcohol: R₂CHOH — oxidised to ketone.

  仲醇：R₂CHOH — 氧化生成酮。

• Tertiary alcohol: R₃COH — not easily oxidised.

  叔醇：R₃COH — 不易被氧化。

With acidified K₂Cr₂O₇, primary and secondary alcohols turn the orange dichromate solution green. Dehydration of alcohols with hot concentrated H₂SO₄ produces alkenes.

在酸化K₂Cr₂O₇存在下，伯醇和仲醇能使橙色重铬酸盐溶液变为绿色。醇在热的浓H₂SO₄作用下脱水生成烯烃。

CH₃CH₂OH → CH₂=CH₂ + H₂O

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4. Haloalkanes: The R-X Group | 卤代烃

Haloalkanes contain a halogen atom (F, Cl, Br, I) bonded to an sp³ carbon. The carbon-halogen bond is polar, making the carbon atom electron-deficient and susceptible to nucleophilic attack.

卤代烃含有与sp³碳原子键连的卤素原子（F、Cl、Br、I）。碳-卤键是极性的，使碳原子缺电子，容易受到亲核试剂进攻。

Nucleophilic substitution: heating with aqueous NaOH produces an alcohol.

亲核取代：与NaOH水溶液共热生成醇。

CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

Elimination: heating with ethanolic NaOH produces an alkene.

消除反应：与NaOH乙醇溶液共热生成烯烃。

CH₃CH₂Br + OH⁻ → CH₂=CH₂ + H₂O + Br⁻

The rate of substitution increases down the halogen group: R-I > R-Br > R-Cl, because the C-I bond is weaker.

取代反应速率随卤素原子序数增大而加快：R-I > R-Br > R-Cl，因为C-I键更弱。

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5. Aldehydes and Ketones: The C=O Group | 醛与酮：羰基

Aldehydes (RCHO) have at least one hydrogen atom attached to the carbonyl carbon, while ketones (R₂CO) have two carbon groups attached. Both contain the polar C=O bond and undergo nucleophilic addition.

醛（RCHO）的羰基碳上至少连有一个氢原子，而酮（R₂CO）的羰基碳上连有两个碳基团。两者都含有极性C=O键，可发生亲核加成。

• Tollens’ reagent (ammoniacal AgNO₃): aldehydes form a silver mirror; ketones do not react.

  托伦试剂（氨性AgNO₃）：醛产生银镜；酮不反应。

• Fehling’s solution: aldehydes give a brick-red Cu₂O precipitate; ketones do not react.

  斐林试剂：醛产生砖红色Cu₂O沉淀；酮不反应。

• Reduction with NaBH₄ or LiAlH₄: aldehydes form primary alcohols; ketones form secondary alcohols.

  用NaBH₄或LiAlH₄还原：醛生成伯醇，酮生成仲醇。

RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 2H₂O + 2NH₃

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6. Carboxylic Acids: The -COOH Group | 羧酸

Carboxylic acids contain the -COOH group, which includes a carbonyl and a hydroxyl group on the same carbon. They are weak acids because the -OH bond is polarised, and the carboxylate anion is stabilised by resonance.

羧酸含有-COOH基团，即在同一个碳上同时含有羰基和羟基。它们都是弱酸，因为-OH键被极化，且羧酸根阴离子通过共振稳定。

Acid reactions:

酸性反应：

• With reactive metals: 2RCOOH + 2Na → 2RCOONa + H₂.

  与活泼金属反应：2RCOOH + 2Na → 2RCOONa + H₂。

• With bases: RCOOH + NaOH → RCOONa + H₂O.

  与碱反应：RCOOH + NaOH → RCOONa + H₂O。

• With carbonates: 2RCOOH + Na₂CO₃ → 2RCOONa + H₂O + CO₂.

  与碳酸盐反应：2RCOOH + Na₂CO₃ → 2RCOONa + H₂O + CO₂。

Carboxylic acids are identified by effervescence with NaHCO₃ and by their sharp, sour odour.

羧酸可通过与NaHCO₃反应产生气泡以及刺激性酸味来鉴别。

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7. Esters: The -COOR Group | 酯

Esters are formed by condensation reaction between a carboxylic acid and an alcohol, with concentrated H₂SO₄ as catalyst. The general structure is RCOOR’.

酯由羧酸和醇在浓H₂SO₄催化下通过缩合反应生成，通式为RCOOR’。

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Esters are volatile compounds with sweet, fruity smells and are used in flavourings and perfumes. They undergo hydrolysis in acid or base; base hydrolysis (saponification) is irreversible and produces a carboxylate salt and an alcohol.

酯是易挥发且具有甜味、果香味的化合物，用于调味和香料。酯可发生酸性或碱性水解；碱性水解（皂化）不可逆，生成羧酸盐和醇。

Identification: The fruity smell, often detected after warming with water and a little H₂SO₄, helps distinguish simple esters.

识别：酯通常具有果香味，可通过与水及少量H₂SO₄加热后闻到香味来辅助鉴别。

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8. Amines: The -NH₂ Group | 胺

Amines are organic derivatives of ammonia, with the general formula RNH₂ (primary amine). The lone pair on nitrogen makes amines basic and nucleophilic.

胺是氨的有机衍生物，伯胺通式为RNH₂。氮原子上的孤对电子使胺具有碱性和亲核性。

• Base properties: RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻.

  碱性：RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻。

• Reaction with acid: RNH₂ + HCl → RNH₃⁺Cl⁻ (ammonium salt).

  与酸反应：RNH₂ + HCl → RNH₃⁺Cl⁻（铵盐）。

• Acylation: amines react with acid chlorides to form amides.

  酰化：胺与酰氯反应生成酰胺。

Aromatic amines, such as phenylamine, are weaker bases than aliphatic amines because the lone pair is delocalised into the benzene ring.

芳香胺（如苯胺）的碱性弱于脂肪胺，因为氮上孤对电子离域到苯环中。

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9. Amides and Nitriles | 酰胺与腈

Amides (RCONH₂) are derived from carboxylic acids and ammonia or amines. They are neutral or very weakly basic, and are hydrolysed by acid or base to give carboxylic acids or carboxylate salts plus ammonia/amine.

酰胺（RCONH₂）由羧酸与氨或胺衍生而来。它们呈中性或极弱碱性，可被酸或碱水解生成羧酸或羧酸盐以及氨/胺。

Nitriles (RC≡N) are made by nucleophilic substitution: R-X + CN⁻ → R-C≡N + X⁻. The nitrile can be reduced to a primary amine or hydrolysed to a carboxylic acid.

腈（RC≡N）可通过亲核取代制备：R-X + CN⁻ → R-C≡N + X⁻。腈可被还原成伯胺，也可水解成羧酸。

CH₃CH₂Br + CN⁻ → CH₃CH₂C≡N + Br⁻

This reaction is useful for extending the carbon chain by one carbon atom.

该反应可用于将碳链延长一个碳原子。

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10. Summary of Chemical Tests | 化学鉴别总结

The table below summarises common identifications for functional groups covered in IB Chemistry. Always state the reagent, condition, and observation in exam answers.

下表总结了IB化学中常见官能团的鉴别方法。答题时请务必说明试剂、条件和现象。

Functional Group	Reagent / Test	Observation
Alkene (C=C)	Br₂ water	Orange to colourless
Alcohol (R-OH)	Acidified K₂Cr₂O₇	Orange to green
Aldehyde (RCHO)	Tollens’ reagent	Silver mirror
Carboxylic acid (-COOH)	NaHCO₃	Fizzing / CO₂ gas
Ester (RCOOR’)	Smell after warming	Sweet fruity odour

In the IB exam, you may be asked to deduce the functional group from infrared (IR) spectra. For example: a broad absorption near 3200-3600 cm⁻¹ suggests O-H in alcohols or carboxylic acids; a sharp peak near 1700 cm⁻¹ suggests C=O; and a peak near 2200 cm⁻¹ suggests C≡N.

在IB考试中，考生可能需要根据红外光谱推断官能团。例如：3200-3600 cm⁻¹附近的宽吸收峰提示醇或羧酸中的O-H；1700 cm⁻¹附近的尖峰提示C=O；2200 cm⁻¹附近的峰提示C≡N。

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