Comprehensive NCEA Level 2 Organic Chemistry Study Guide

Alkenes: Structure and Properties

• Definition: Alkenes are hydrocarbons that possess one or more carbon-to-carbon double bonds ($C=C$).

• Unsaturation: They are unsaturated compounds, meaning they can form more bonds by breaking the $C=C$ double bond and allowing other atoms to form single bonds with the carbon atoms.

• Reactivity: Alkenes are significantly more reactive than alkanes.

• Naming Branched Alkenes:

  • Identify the longest carbon chain containing the double bond.

  • Number the chain so the double bond has the lowest possible number.

  • Examples: $3\text{-methylpent-2-ene}$, $2,3\text{-dimethylpent-2-ene}$.

Geometric Isomers

• Theory: The carbon-to-carbon double bond prevents the free rotation of the atoms or groups attached to it. This means the groups attached to each side of the $C=C$ bond are fixed in space.

• Types:

  • $cis\text{ isomers}$: Identical groups are located on the same side of the double bond.

  • $trans\text{ isomers}$: Identical groups are located on opposite sides of the double bond.

• Examples (But-2-ene):

  • $cis\text{-but-2-ene}$: Methyl groups are on the same side.

  • $trans\text{-but-2-ene}$: Methyl groups are diagonally opposite.

• Requirements for Geometric Isomerism:

  1. Must contain a carbon-to-carbon double bond ($C=C$) to prevent rotation.

  2. Each carbon atom involved in the double bond must have two different atoms or groups attached to it.

Testing for Double Bonds (Unsaturation)

• Method: Halogens, specifically bromine water ($Br_2(aq)$), are used to test for the presence of a double bond.

• Observation (Alkene): In the presence of an unsaturated compound, an addition reaction occurs, and the orange bromine water decolorizes almost instantly.

• Observation (Alkane): If the compound is saturated, the orange color will not decolorize unless the mixture is placed under UV light, which initiates a slow substitution reaction.

Addition Reactions of Alkenes

• Combustion: Alkenes react with oxygen ($O_2$) to produce carbon dioxide ($CO_2$) and water ($H_2O$).

• Hydrogenation:

  • Reaction: Addition of $H_2$.

  • Catalyst: Platinum ($Pt$) or Nickel ($Ni$) is required.

  • Product: Alkane.

• Halogenation:

  • Reaction: Addition of halogens ($X_2$), such as $Cl_2$ or $Br_2$.

  • Example: Ethane + $Br_2(aq)$ (orange) \rightarrow $1,2\text{-dibromoethane}$ (colorless).

• Hydrohalogenation:

  • Reaction: Addition of hydrogen halides ($HX$), such as $HCl$ or $HBr$.

  • Product: Halogenoalkane (haloalkane).

• Hydration:

  • Reagents: Steam and pressure with an acid catalyst ($diluted H_2SO_4$ or acidified water $H_2O/H^+$).

  • Reaction: Water is added across the double bond.

  • Product: Alcohol.

• Oxidation Reaction (Diol Formation):

  • Reagent: Aqueous potassium permanganate ($KMnO_4(aq)$).

  • Observations:

  1. Neutral: Purple $KMnO_4(aq)$ turns into a brown solid ($MnO_2$).

  2. Acidic ($H^+$): Purple $KMnO_4(aq)$ solution is decolorized to form colorless $Mn^{2+}$ ions.

  • Product: A diol (e.g., $ethene\text{-1,2-diol}$).

Markovnikov's Rule

• Context: Applied to addition reactions with asymmetrical alkenes.

• Symmetrical vs. Asymmetrical:

  • Symmetrical alkenes yield only one possible product.

  • Asymmetrical alkenes yield two possible products: the Major product (most common) and the Minor product (least common).

• The Rule ("The reach get richer"): The carbon atom in the double bond that already has the most hydrogen atoms attached will receive the new hydrogen atom, while the other carbon atom in the double bond (with fewer hydrogens) will receive the halogen or hydroxyl group.

• Example: Propene + $HBr$

  • Major Product: $2\text{-bromopropane}$.

  • Minor Product: $1\text{-bromopropane}$.

Haloalkanes: Classification and Naming

• Definition: Alkanes where one or more hydrogens are replaced by group 17 elements (halogens: $F, Cl, Br, I$).

• Naming Prefixes: "fluoro-", "chloro-", "bromo-", "iodo-".

• Example Names: $2\text{-bromopropane}$, $1\text{-iodopentane}$, $1,1\text{-dichloroethene}$, $1,2\text{-dichloroethane}$, $1\text{-bromo-2-chloropropane}$.

• Classification:

  • Primary ($1^\circ$): The carbon with the halogen is attached to one other carbon.

  • Secondary ($2^\circ$): The carbon with the halogen is attached to two other carbons.

  • Tertiary ($3^\circ$): The carbon with the halogen is attached to three other carbons.

Reactions of Haloalkanes

• Substitution with $KOH(aq)$: The halogen atom is replaced by a hydroxyl ($OH$) group to form an alcohol.

• Substitution with $NH_3(alc)$: The halogen atom is substituted for an amine ($NH_2$) group to form an amine and ammonium halide ($NH_4X$).

• Elimination with $conc. KOH(alc)$: Known as dehydrohalogenation. The halogen atom and a hydrogen atom from a neighboring carbon are removed to form a $C=C$ double bond (alkene).

Alcohols: Properties and Synthesis

• Structure: Compounds containing at least one hydroxyl ($OH$) group. General formula: $C_nH_{2n+1}OH$.

• Naming: Suffix "-ol" (e.g., $methanol$, $ethanol$, $propan-2-ol$).

• Polyalcohols:

  • Diols: Compounds with two $OH$ groups (e.g., $ethan\text{-1,2-diol}$, $propan\text{-1,2-diol}$).

  • Triols: Compounds with three $OH$ groups (e.g., $propan\text{-1,2,3-triol}$).

• Physical Properties:

  • Melting and Boiling Points: Increase with chain length due to stronger intermolecular forces. Generally higher than alkanes of similar mass due to the polar $OH$ group.

  • Solubility: Small alcohols are soluble in water (hydrophilic polar head). Solubility decreases as the non-polar alkyl chain length (hydrophobic tail) increases.

• Alcohol Reactions:

  • Substitution with $HX$: Forms a haloalkane and water.

  • Substitution with halogenating reagents ($PCl_3, PCl_5, SOCl_2$): Replaces $OH$ with a halogen.

  • Oxidation: Primary alcohols oxidize to carboxylic acids using acidified potassium permanganate ($KMnO_4/H^+$) or acidified dichromate ($Cr_2O_7^{2-}/H^+$).

  • Elimination (Dehydration): Uses $conc. H_2SO_4$ to remove $H_2O$ and form an alkene.

Saytzeff's Rule (Elimination Rule)

• Context: Applied during dehydration of asymmetrical alcohols.

• The Rule ("The poor get poorer"): The hydrogen atom is most likely to be removed from the neighboring carbon atom that has the fewest hydrogen atoms already attached.

• Products:

  • Major Product: Double bond forms toward the "poor" side.

  • Minor Product: Double bond forms toward the side with more hydrogens.

• Example: $butan\text{-2-ol}$ elimination.

  • Major product: $but\text{-2-ene}$.

  • Minor product: $but\text{-1-ene}$.

Amines

• Functional Group: Amino group ($NH_2$). Derived from ammonia ($NH_3$).

• Naming: Suffix "-amine" (e.g., $propan\text{-2-amine}$, $hexan\text{-1-amine}$, $2\text{-methylpentan-2-amine}$).

• Physical Properties:

  • High polar bonds due to Nitrogen's electronegativity results in higher MP/BP compared to alkanes.

  • Water-soluble when small, but solubility decreases as carbon chain increases.

• Reactions:

  • Synthesis: Substitution of a haloalkane with alcoholic ammonia ($NH_3(alc)$).

  • Basic Density: Amines form basic solutions in water (turn red litmus blue) by forming ammonium ions.

  • Salt Formation: React with acids ($HCl$) to form ammonium salts ($R\text{-}NH_3^+Cl^-$).

  • Reaction with Carboxylic Acids: Forms salts like $ethylammonium ethanoate$.

Carboxylic Acids

• Functional Group: Carboxyl group ($-COOH$).

• Properties: Weak acids that partially dissociate in water into alkanoate ions and $H^+$.

• Naming: Suffix "-anoic acid" (e.g., $ethanoic acid$, $2,2\text{-dimethylpropanoic acid}$).

• Reactions:

  • Carboxylic Acid + Base \rightarrow Salt + Water.

  • Carboxylic Acid + Metal \rightarrow Salt + Hydrogen Gas ($H_2$).

  • Carboxylic Acid + Metal Carbonate \rightarrow Salt + $CO_2$ (bubbles) + Water.

  • Carboxylic Acid + Amine \rightarrow Ammonium Salt.

Alkynes and Polymerization

• Alkynes:

  • Structure: Hydrocarbons with triple bonds ($C \equiv C$). General formula: $C_nH_{2n-2}$.

  • Reactions: Undergo two-step addition hydrogenation or halogenation (Alkyne \rightarrow Alkene \rightarrow Alkane/Haloalkane).

• Polymerization:

  • Definition: Large molecules made of repeating units (monomers).

  • Addition Polymerization: Alkenes break their double bonds to link into long chains under heat and catalyst conditions.

Alkanes and Cyclic Compounds

• Alkanes: Saturated hydrocarbons with single bonds. Low reactivity. Substitution occurs with halogens under UV light.

• State Change with Length:

  • $C_1$ to $C_4$: Gases at room temperature ($25\,^\circ C$).

  • $C_5$ to $C_{15}$: Liquids at room temperature.

• Cyclic Alkanes: Alkanes in an acyclic ring structure ($n \ge 3$). General formula: $C_nH_{2n}$. Slightly higher melting and boiling points than linear alkanes.

• Structural Isomers: Compounds with the same molecular formula but different atomic arrangements (e.g., hexane, 2-methylpentane, 2,2-dimethylbutane).

Questions & Discussion

• Do Now Exercises:

  • Name the compound with a 5-carbon chain and a methyl branch on the 3rd carbon: $3\text{-methylpentane}$.

  • Define structural isomers: Compounds with the same molecular formula but atoms arranged/connected differently.

• Testing unknown substances: If a substance decolorizes orange bromine water instantly, it is an alkene. If it requires UV light and is slow, it is an alkane. If bubbles are produced when added to a carbonate, it is a carboxylic acid.