C5H8 Draw structure with Two double bonds Is this an alkene or an alkyne? Hill formula and structure Lewis Structure Draw structure with Triple bond Is this an alkene or alkyne? Hill formula and structure Lewis Structure One double bond and one ring Hill formula and structure Lewis Structure Alkene or alkyne 10- Five carbons with nitrogen Hill formula with nitrogen. Use this for all three structures Primary amine Secondary amine Tertiary amine – Ethane with bromine Hill formula
The Correct Answer and Explanation is:
A. Isomers of C₅H₈
| # | Condensed (Lewis) structure* | Hill formula | Unsaturation | Alkene / Alkyne |
|---|---|---|---|---|
| 1 | CH₂=CH-CH=CH-CH₃ ( 1,3-pentadiene ) | C₅H₈ | two C=C | alkene |
| 2 | CH≡C-CH₂-CH₂-CH₃ ( 1-pentyne ) | C₅H₈ | one C≡C | alkyne |
| 3 | C₅H₈ | ring + one C=C | alkene |
*Each line shows all valence bonds; lone-pair electrons on carbon are understood.
B. Five-carbon amines (C₅H₁₃N)
| Amine class | Example condensed formula (Lewis skeleton) | Hill formula |
|---|---|---|
| Primary (1°) | CH₃-CH₂-CH₂-CH₂-CH₂-NH₂ (1-aminopentane) | C₅H₁₃N |
| Secondary (2°) | CH₃-CH₂-NH-CH₂-CH₂-CH₃ (ethyl-n-propylamine) | C₅H₁₃N |
| Tertiary (3°) | (CH₃)₂N-CH₂-CH₂-CH₃ (N,N-dimethyl-n-propylamine) | C₅H₁₃N |
C. Haloalkane derived from ethane
- Bromoethane (ethyl bromide)
Lewis sketch: CH₃-CH₂-Br
Hill formula: C₂H₅Br
explanation
Structural isomers differ only in the way atoms are connected, so the same Hill formula can hide very different functional behaviour. The degree of unsaturation (DoU) helps decide what is possible: DoU = C – H/2 + N/2 + 1 (oxygen and halogens are ignored). For C₅H₈, DoU = 2, so each isomer must contain any combination that adds up to two units of unsaturation.
1,3-Pentadiene spends them on two C=C bonds, qualifying it as an alkene (specifically a diene). 1-Pentyne invests both units in a single C≡C bond, making it an alkyne. Cyclopentene uses one ring (one DoU) and one C=C bond (another), so although it is cyclic it is still an alkene. These differences alter reactivity: dienes undergo Diels-Alder cycloadditions, alkynes add electrophiles in a two-step process, while cycloalkenes suffer ring strain that can change their selectivity.
Substituting one hydrogen in a five-carbon skeleton with nitrogen gives C₅H₁₃N (DoU = 0). If the nitrogen bears one carbon it is a primary amine (1-aminopentane), two carbons give a secondary amine (ethyl-n-propylamine), and three carbons a tertiary amine (N,N-dimethyl-n-propylamine). The protonation behaviour (pKₐ) of the conjugate acids rises modestly from primary to tertiary because extra alkyl groups stabilise the positive charge by induction, yet steric hindrance can slow reactions that require the lone pair to approach an electrophile.
Halogenation illustrates Hill ordering: after radical substitution of Br• into ethane the product is bromoethane. Its empirical listing is C₂H₅Br – carbon first, hydrogen second, all other elements alphabetically thereafter – regardless of how the structural formula is written (CH₃-CH₂-Br) or how the compound is named. Mastering these conventions allows chemists to sketch correct Lewis structures and to predict quickly whether a molecule behaves as an alkane, alkene, alkyne, or amine.
