Hydrocarbon Derivatives: Functional Groups and Their Reactions

What Are Hydrocarbon Derivatives?

Hydrocarbon derivatives are organic molecules where one or more hydrogen atoms in a hydrocarbon have been replaced by a functional group — a specific atom or group of atoms that determines the molecule's chemical behaviour.

The functional group is the "personality" of the molecule. While the carbon backbone provides structure, the functional group dictates reactivity, polarity, boiling point, and how the molecule interacts with others.

Learning Goals: By the end of this guide, you should be able to:

Identify and name the major functional groups.

Predict physical properties from functional group type.

Understand the characteristic reactions of each group.

Classify functional groups by oxidation level.

Apply these concepts to multi-step synthesis problems.

The Major Functional Groups

Alcohols ( $–OH$ )

The hydroxyl group makes alcohols polar and capable of hydrogen bonding, giving them higher boiling points than similar-sized hydrocarbons.

Type	Structure	Example	Boiling Point
Primary (1°)	$RCH_2OH$	Ethanol	78°C
Secondary (2°)	$R_2CHOH$	Propan-2-ol	82°C
Tertiary (3°)	$R_3COH$	2-methylpropan-2-ol	83°C

Key reactions: Oxidation (to aldehyde/ketone/acid), dehydration (to alkene), esterification (with carboxylic acid).

Aldehydes ( $–CHO$ ) and Ketones ( $>C=O$ )

Both contain the carbonyl group ( $C=O$ ), but their position differs:

Feature	Aldehyde	Ketone
Carbonyl position	End of chain	Middle of chain
General formula	$RCHO$	$RCOR'$
Example	Ethanal ( $CH_3CHO$ )	Propanone ( $CH_3COCH_3$ )
Oxidation	Can be oxidised → carboxylic acid	Cannot be easily oxidised
Tollen's reagent	Positive (silver mirror)	Negative

Carboxylic Acids ( $–COOH$ )

The carboxyl group combines $C=O$ and $O–H$ , making these molecules acidic. They donate $H^+$ to bases:

RCOOH + NaOH \rightarrow RCOONa + H_2O

Carboxylic acids have the highest boiling points among functional groups of similar size because they form two hydrogen bonds between molecules (dimerisation).

Esters ( $–COO–$ )

Formed from the reaction of a carboxylic acid with an alcohol:

RCOOH + R'OH \xrightleftharpoons{H^+} RCOOR' + H_2O

Esters are responsible for many fruity smells and are used in perfumes, flavourings, and plasticisers. They have lower boiling points than corresponding acids or alcohols because they cannot hydrogen bond with themselves.

Amines ( $–NH_2$ )

Amines contain nitrogen and act as bases (lone pair on N accepts $H^+$ ):

RNH_2 + HCl \rightarrow RNH_3^+Cl^-

They have a characteristic fishy smell and can form hydrogen bonds (N–H), giving them moderate boiling points.

Master Comparison Table

Functional Group	Formula	Suffix	Polarity	H-Bonding	Boiling Point
Alcohol	$–OH$	-ol	Polar	Yes (strong)	High
Aldehyde	$–CHO$	-al	Polar	Acceptor only	Medium
Ketone	$>C=O$	-one	Polar	Acceptor only	Medium
Carboxylic acid	$–COOH$	-oic acid	Very polar	Yes (dimers)	Highest
Ester	$–COO–$	-oate	Polar	No (self)	Low-Medium
Amine	$–NH_2$	-amine	Polar	Yes (moderate)	Medium

Functional Group Explorer

Visualise 3D models of alcohols, aldehydes, ketones, carboxylic acids, and more. See how functional groups change molecular shape and properties.

View 12+ Derivatives in 3D

The Oxidation Ladder

Organic chemistry has a beautiful oxidation sequence connecting functional groups:

\text{Alcohol} \xrightarrow{[O]} \text{Aldehyde} \xrightarrow{[O]} \text{Carboxylic Acid}

Starting Material	Oxidation Product	Reagent
Primary alcohol	Aldehyde (distil) or Carboxylic acid (reflux)	$K_2Cr_2O_7 / H_2SO_4$
Secondary alcohol	Ketone	$K_2Cr_2O_7 / H_2SO_4$
Tertiary alcohol	No reaction	Resistant to oxidation

Visual indicator: Acidified potassium dichromate turns from orange to green when oxidation occurs.

Worked Examples

Example 1: Identifying Functional Groups

Question: Identify all functional groups in aspirin ( $C_9H_8O_4$ ): it contains a benzene ring, an ester group, and a carboxylic acid group.

Solution: Aspirin has:

Carboxylic acid ( $–COOH$ ) → makes it acidic, soluble in NaOH
Ester ( $–COO–$ ) → can be hydrolysed back to salicylic acid
Aromatic ring (benzene) → provides structural stability

Example 2: Predicting Boiling Points

Question: Rank in order of increasing boiling point: propanal ( $CH_3CH_2CHO$ ), propan-1-ol ( $CH_3CH_2CH_2OH$ ), propanoic acid ( $CH_3CH_2COOH$ ).

Solution: All have 3 carbons and similar molecular mass.

Propanal: carbonyl is an H-bond acceptor but not donor → lowest BP
Propan-1-ol: O–H can donate and accept H-bonds → medium BP
Propanoic acid: forms H-bonded dimers → highest BP

Order: propanal < propan-1-ol < propanoic acid ✓

Example 3: Distinguishing Aldehydes from Ketones

Question: How do you test whether an unknown carbonyl compound is an aldehyde or a ketone?

Solution: Use Tollen's reagent ( $Ag(NH_3)_2^+$ , ammoniacal silver nitrate):

Warm the unknown compound with Tollen's reagent.
Aldehyde: produces a silver mirror on the test tube wall (positive test).
Ketone: no reaction (negative test).

This works because aldehydes can be oxidised, reducing $Ag^+$ to metallic $Ag$ .

Common Mistakes

Confusing aldehydes and ketones — Both have $C=O$ , but aldehydes are at the end of the chain (at least one H on the carbonyl carbon), ketones are in the middle (two R groups on the carbonyl carbon).
Thinking esters are acids — Esters ( $–COO–$ ) look similar to carboxylic acids ( $–COOH$ ) but they are not acidic because they lack the $O–H$ group.
Forgetting that tertiary alcohols resist oxidation — Only primary and secondary alcohols can be oxidised by acidified dichromate. Tertiary alcohols have no hydrogen on the carbon bearing the $–OH$ .
Mixing up esterification direction — In $RCOOH + R'OH \rightleftharpoons RCOOR' + H_2O$ , the acid provides the $C=O$ part and the alcohol provides the $–O–R'$ part of the ester.

Exam Tips (A-Level / AP / IB)

Name the functional group precisely: Don't just say "it has an oxygen" — state "hydroxyl group" or "carbonyl group" or "carboxyl group."
For boiling point comparisons: hydrogen bonding capability > polarity > molecular size.
Know the three common tests: Tollen's (aldehydes), bromine water (alkenes), acidified dichromate (alcohols).
For multi-step synthesis, always think about the oxidation ladder: alcohol → aldehyde → acid.

Frequently Asked Questions

What determines the reactivity of a functional group?

The type and arrangement of atoms in the functional group determine its electron density, polarity, and ability to interact with reagents. Groups with electron-rich areas (lone pairs, $\pi$ bonds) are more reactive toward electrophiles.

Why do carboxylic acids have higher boiling points than alcohols?

Carboxylic acids form hydrogen-bonded dimers — pairs of molecules linked by two H-bonds each. This effectively doubles the intermolecular attraction, requiring much more energy to separate the molecules.

Can a molecule have more than one functional group?

Yes. Many biological molecules have multiple functional groups. Amino acids have both $–NH_2$ (amine) and $–COOH$ (carboxylic acid). The interactions between multiple groups create complex and unique chemistry.

What is the difference between esterification and hydrolysis?

Esterification combines an acid and alcohol to form an ester (forward reaction). Hydrolysis breaks an ester back into its component acid and alcohol using water (reverse reaction). Both are reversible.

Hydrocarbons — The parent molecules from which derivatives are formed.
Structural Isomerism — Different functional group isomers with the same molecular formula.
Curly Arrow Mechanisms — The electron-pushing notation that explains how these reactions happen.