Hydrocarbons For 9th And 10th

• Alkyne - hydrocarbon with at least one C≡C triple bond; general formula CnH2n-2.

• Longest carbon chain - the continuous sequence of carbon atoms used as the parent for naming.

• IUPAC naming (parent chain + numbering) - rules for choosing the parent chain and numbering to give the lowest bond position.

• Bond position number - the number showing where the C=C or C≡C occurs in the numbered chain.

Active Recall

• Hydrocarbon = __________________________________________________

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• Alkane = __________________________________________________

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• Alkene = __________________________________________________

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• Alkyne = __________________________________________________

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• Longest carbon chain = __________________________________________________

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• IUPAC naming (parent chain + numbering) = __________________________________________________

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• Bond position number = __________________________________________________

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Worked Examples

Example 1: Identify the type from bonding

Given: A chain with one C=C double bond and no triple bonds.

• Look for bond types between carbon atoms.

• A C=C means it is an alkene (ending will be -ene ).

• If there is only one double bond and no triple bonds, name will be based on the longest chain and the double-bond position.

Now you try: A hydrocarbon has a C≡C triple bond and no double bonds. What type is it?

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Example 2: Name a simple alkane from a diagram (straight chain)

Given: A straight chain of 5 carbon atoms with only single bonds.

• Find the longest carbon chain: 5 carbons → base name pent-.

• Only single bonds → alkane → ending -ane.

• No bond position needed because there are no double/triple bonds.

• Name = pentane.

Now you try: A straight chain of 3 carbon atoms with only single bonds. Name it.

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Example 3: Name an alkene using correct numbering

Given: A 6-carbon chain with a double bond between carbon 2 and 3 (from the left end).

• Longest chain length = 6 → base name hex-.

• Double bond → -ene.

• Bond position = 2 (written as but/hex/etc.-2-ene style).

• Name = hex-2-ene.

• If the double bond were also “2” from the other end, numbering choice would not change; if different, choose the lowest number.

Now you try: A 4-carbon chain has a double bond between carbon 1 and 2 (as drawn from one end). What is the correct name?

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Practice Questions

1) (Easy) Classification

State whether each is an alkane, alkene, or alkyne based on the carbon-carbon bonds.

a) Only single bonds

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b) One double bond (C=C)

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c) One triple bond (C≡C)

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2) (Easy) Base name from chain length

A straight chain has 7 carbon atoms and only single bonds. Give the IUPAC-style name.

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3) (Medium) Longest chain rule

A molecule contains a branched carbon skeleton shown as: a main path of 5 connected carbons, with a side branch of 2 carbons. There are only single bonds.

a) What is the longest carbon chain length?

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b) What is the IUPAC-style parent name ending? (e.g., -ane / -ene / -yne)

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4) (Medium) Number the double bond correctly

Name the alkene with a longest chain of 5 carbons and a double bond between carbon 2 and 3 (numbering starts from the left end shown).

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5) (Hard) Choose the lowest set of numbers

A 6-carbon chain has two double bonds. From one end they are at 2 and 5; from the other end they are at 1 and 4.

a) Which numbering gives the lowest set of numbers?

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b) Write the IUPAC-style name.

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6) (Hard) Mixed bond types (single + triple)

A hydrocarbon has a 5-carbon longest chain and one triple bond. The triple bond is between carbon 2 and 3 (from the numbering that gives the lowest position).

Write the IUPAC-style name.

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Answer Key

• a) alkane

b) alkene

c) alkyne

• heptane

• a) 5

b) -ane

• pent-2-ene

• a) 1 and 4

b) hex-1,4-diene

• pent-2-yne

Exam Tips & Common Mistakes

• Confusing bond type with name ending: C=C → -ene, C≡C → -yne, only single bonds → -ane.

• Choosing the wrong longest chain: always pick the continuous chain with the most carbons, not the longest “straight-looking” part.

• Wrong numbering direction: number from the end that gives the lowest position numbers for double/triple bonds.

• Forgetting positions when there’s a double/triple bond: names like hex-2-ene must include the position.

• Mixing up “alkene/alkyne” with “alkane”: alkane has no multiple bonds (only C-C single).

Examiner-focused tips:

• For 4-mark style answers, use correct classification words: alkane/alkene/alkyne plus one

End of chapter one. 4 more chapters in the full book.