Organic functional groups

Chemistry B (Salters) - H033 · Cambridge OCR AS Level · 5 min read · Updated: Mar 26, 2026

Welcome to the Ozone Story: Organic Functional Groups

In this chapter, we are diving into the "Ozone Story" (OZ). This section of your OCR Chemistry B course is all about the chemistry happening high above our heads in the atmosphere. To understand why certain chemicals (like old-fashioned hairspray propellants) were damaging the ozone layer, we first need to understand organic functional groups—specifically haloalkanes and amines.

Don't worry if organic chemistry feels like a new language at first! We’ll break it down step-by-step, starting with how to name these molecules and moving on to how they behave.

1. Identifying the Players: Haloalkanes and Amines

A functional group is a specific atom or group of atoms responsible for the characteristic chemical reactions of a molecule. Think of it as the "personality" of the molecule.

Haloalkanes

These are alkanes where one or more hydrogen atoms have been replaced by a halogen atom (Fluorine, Chlorine, Bromine, or Iodine).
Example: Chloromethane, \(CH_{3}Cl\)

Amines

Amines are derivatives of ammonia (\(NH_{3}\)). In an amine, one or more of the hydrogen atoms in ammonia are replaced by an alkyl group (a carbon chain).
Example: Methylamine, \(CH_{3}NH_{2}\)

Quick Review:
• Haloalkane: Carbon chain + Halogen (F, Cl, Br, I)
• Amine: Carbon chain + Nitrogen group (\(-NH_{2}\))

2. Naming Haloalkanes (Systematic Nomenclature)

To keep chemists across the world on the same page, we use a specific set of rules to name these molecules. Follow these steps:

1. Find the longest carbon chain: This gives you the "parent" name (e.g., 3 carbons = propane).
2. Identify the halogen: Use a prefix: fluoro-, chloro-, bromo-, or iodo-.
3. Number the chain: Start from the end that gives the halogen the lowest possible number.
4. Alphabetical order: If there are different halogens, list them alphabetically (e.g., bromo- comes before chloro-).

Example: \(CH_{3}CHBrCH_{3}\) is 2-bromopropane because the bromine is on the second carbon of a three-carbon chain.

3. Physical Properties: Boiling Points

Why are some haloalkanes gases and others liquids? It all comes down to intermolecular bonds (the "sticky" forces between molecules).

The boiling point of haloalkanes depends on two main things:
• Chain Length: Longer chains have more electrons, leading to stronger instantaneous dipole–induced dipole bonds (often called London forces). More "stickiness" means a higher boiling point.
• The Halogen Type: As you go down Group 7 (from Fluorine to Iodine), the atoms get larger and have more electrons. This significantly increases the strength of the London forces. Iodoalkanes have much higher boiling points than fluoroalkanes of the same chain length.

Did you know? Many CFCs (chlorofluorocarbons) were used specifically because they have low boiling points, making them easy to turn into gases for use in refrigerators!

4. Reactivity: The Tug-of-War

Why do haloalkanes react the way they do? There are two factors competing to decide how "reactive" a molecule is:

Factor A: Bond Polarity

Halogens are more electronegative than carbon. This means they pull the shared electrons in the \(C-X\) bond (where \(X\) is the halogen) toward themselves.
• This creates a polar bond: the Carbon is slightly positive (\(\delta+\)) and the Halogen is slightly negative (\(\delta-\)).
• The \(C-F\) bond is the most polar because Fluorine is the most electronegative. You might think this makes it the most reactive, but wait!

Factor B: Bond Enthalpy (Bond Strength)

Bond enthalpy is the energy needed to break the bond.
• The \(C-F\) bond is very short and very strong.
• The \(C-I\) bond is long and much weaker.
• Crucial Point: Experimental evidence shows that bond enthalpy is more important than polarity. Even though the \(C-I\) bond is the least polar, it is the most reactive because the bond is so easy to break.

Memory Aid: "Weak bond, fast reaction!" Iodoalkanes react fastest because the \(C-I\) bond is the weakest.

5. Nucleophilic Substitution Reactions

In the "Ozone Story," we focus on substitution reactions. This is where one atom or group is swapped for another.

Key Terms:

• Nucleophile: An electron-pair donor. It is "nucleus-loving" and looks for a positive center (\(\delta+\) Carbon) to attack. Examples include \(OH^{-}\), \(H_{2}O\), and \(NH_{3}\).
• Substitution: A reaction where an atom or group is replaced by a different atom or group.

The Reactions You Need to Know:

1. With Hydroxide Ions (\(OH^{-}\)): Forms an alcohol.
\(R-X + OH^{-} \rightarrow R-OH + X^{-}\)
2. With Water (\(H_{2}O\)): Also forms an alcohol, but much slower (water is a weaker nucleophile).
3. With Ammonia (\(NH_{3}\)): Forms an amine.
\(R-X + 2NH_{3} \rightarrow R-NH_{2} + NH_{4}X\)

6. The \(S_{N}2\) Mechanism

The \(S_{N}2\) mechanism is a step-by-step map of how a nucleophile replaces a halogen. "S" stands for substitution, "N" for nucleophilic, and "2" because two molecules are involved in the initial collision.

Step-by-Step Mechanism (e.g., \(OH^{-}\) attacking Chloromethane):

1. The nucleophile (\(OH^{-}\)) approaches the \(\delta+\) Carbon from the side opposite the halogen.
2. A "curly arrow" is drawn from the lone pair on the Oxygen of the \(OH^{-}\) to the \(\delta+\) Carbon. This shows a new bond forming.
3. Simultaneously, a second curly arrow is drawn from the \(C-Cl\) bond to the Chlorine atom. This shows the bond breaking and the electrons moving to the Chlorine.
4. The Chlorine leaves as a halide ion (\(Cl^{-}\)), and the \(OH\) is now bonded to the Carbon.

Note: Always include partial charges (\(\delta+\) and \(\delta-\)) on your diagram to show you understand bond polarity!

Key Takeaway: The \(S_{N}2\) reaction happens in one smooth motion—as the new bond forms, the old one breaks!

Summary: The "Big Ideas" for your Exam

• Haloalkanes are named with prefixes (chloro-, bromo-, etc.) based on the halogen present.
• Boiling points increase with chain length and as you go down the halogen group (due to stronger London forces).
• Reactivity is determined by bond enthalpy. Iodoalkanes are the most reactive because the \(C-I\) bond is the weakest.
• Nucleophiles (like \(OH^{-}\) or \(NH_{3}\)) attack the \(\delta+\) carbon in a substitution reaction.
• The \(S_{N}2\) mechanism uses curly arrows to show the movement of electron pairs during the reaction.

References

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the definition of a functional group in organic chemistry?

A functional group is a specific atom or group of atoms in a molecule that is responsible for its characteristic chemical reactions.

What constitutes a haloalkane?

A haloalkane is an alkane where one or more hydrogen atoms have been replaced by a halogen atom, such as Fluorine (F), Chlorine (Cl), Bromine (Br), or Iodine (I).

How are amines defined in organic chemistry?

Amines are derivatives of ammonia (\(NH_{3}\)) in which one or more hydrogen atoms are replaced by alkyl groups (carbon chains).

What are the key rules for numbering and ordering halogens when naming haloalkanes?

The carbon chain is numbered to give the halogen the lowest possible position number, and different halogens are listed alphabetically as prefixes.

How do chain length and halogen type affect the boiling points of haloalkanes?

Boiling points increase with longer carbon chains and heavier halogens because they possess more electrons, leading to stronger instantaneous dipole–induced dipole bonds (London forces).

Why is the carbon–halogen bond in a haloalkane described as polar?

Halogens are more electronegative than carbon, so they pull the shared electrons toward themselves, creating a partial positive charge (\(\delta+\)) on the carbon and a partial negative charge (\(\delta-\)) on the halogen.

Which factor is more important in determining haloalkane reactivity: bond polarity or bond enthalpy?

Bond enthalpy is more important; iodoalkanes are the most reactive because the \(C-I\) bond is the weakest (lowest enthalpy), making it the easiest to break.

What is a nucleophile?

A nucleophile is an electron-pair donor that is attracted to electron-deficient (\(\delta+\)) centers, such as the carbon atom in a polar bond.

What occurs during a nucleophilic substitution reaction between a haloalkane and a hydroxide ion?

The hydroxide ion (\(OH^{-}\)) replaces the halogen atom, resulting in the formation of an alcohol and a halide ion.

What are the products when a haloalkane reacts with ammonia?

The primary organic product is an amine, and the inorganic byproduct is an ammonium halide salt.

What does the '2' in the \(S_{N}2\) mechanism signify?

It stands for 'bimolecular', meaning that two species (the nucleophile and the haloalkane) are involved in the collision that determines the reaction rate.

What is the function of a "curly arrow" in a reaction mechanism diagram?

A curly arrow represents the movement of a pair of electrons during a chemical reaction.

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