2.8.1Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Halogenoalkanes – Introduction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 46 of 53 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Naming Halogenoalkanes
Halogenoalkanes are alkanes in which one or more hydrogen atoms are replaced by halogen atoms.
1
Find the longest carbon chain and name it as the parent alkane, such as propane for three carbons.
2
Use the prefixes fluoro-, chloro-, bromo- or iodo- for the halogen atoms.
3
Number the carbon chain so the substituent has the lowest possible number, giving 1-bromopropane or 2-bromopropane.
4
List multiple substituents in alphabetical order, so CH₃CHClCH₂Br is 1-bromo-2-chloropropane.
5
Include di- or tri- when a halogen appears more than once, but ignore these prefixes when ordering alphabetically.
Section 2 of 5Classification and Structure
Primary
- Primary The carbon bonded to the halogen is attached to one other carbon, as in 1-bromopropane.
Secondary
- Secondary The carbon bonded to the halogen is attached to two other carbons, as in 2-bromopropane.
Tertiary
- Tertiary The carbon bonded to the halogen is attached to three other carbons, as in 2-chloro-2-methylpropane.
Functional group
- Functional group The C-X bond is the functional group, where X is F, Cl, Br or I.
Polarity
- Polarity Halogens are more electronegative than carbon, so the carbon is δ⁺ and the halogen is δ⁻.
| Compound | Classification |
|---|---|
| CH₃CH₂CH₂Br | Primary |
| CH₃CHBrCH₃ | Secondary |
| (CH₃)₃CCl | Tertiary |
Section 3 of 5Properties and Uses
Properties and Uses
Boiling point
- Boiling point Boiling points are higher than those of the matching alkanes and rise with chain length and with halogen size.
Solubility
- Solubility They are only slightly soluble in water because they cannot form hydrogen bonds, but dissolve in organic solvents.
Reactivity
- Reactivity C-F is strongest and C-I is weakest, so iodoalkanes usually react fastest.
Uses
- Uses They are used as solvents, in making polymers such as PVC and PTFE, and in the synthesis of other organic compounds.
Section 4 of 5Worked Example
Question Name CH₃CHBrCH₂CH₃ and (CH₃)₃CCl, and state whether each is primary, secondary or tertiary.
- CH₃CHBrCH₂CH₃ has a four-carbon chain with bromine on carbon 2, so it is 2-bromobutane.
- The carbon bonded to Br is attached to two other carbons, so it is secondary.
- (CH₃)₃CCl has a three-carbon chain with a methyl group and chlorine both on carbon 2, so it is 2-chloro-2-methylpropane.
- The carbon bonded to Cl is attached to three carbons, so it is tertiary.
Answer
- 2-bromobutane is secondary;
- 2-chloro-2-methylpropane is tertiary.
Section 5 of 5Exam Checklist & Tips
2.8.1 – Key Points for the Exam
- Halogenoalkane An organic compound in which one or more hydrogen atoms of an alkane are replaced by halogen atoms.
- Primary halogenoalkane A halogenoalkane in which the C-X carbon is bonded to one other carbon.
- Tertiary halogenoalkane A halogenoalkane in which the C-X carbon is bonded to three other carbons.
- Carbon-halogen bond The polar C-X bond that is the functional group of halogenoalkanes.
- Bond enthalpy The energy needed to break a particular covalent bond.
Common Mistakes
- Counting the carbons in the whole molecule instead of the carbons attached to the C-X carbon when classifying.
- Listing substituents in the wrong order, or numbering from the wrong end of the chain.
- Saying halogenoalkanes are very soluble in water because the C-X bond is polar.
- Writing that C-F reacts fastest because it is the most polar; its stronger bond makes it the slowest.
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