Comprehensive Study Guide for Hydrocarbons, Carbonyls, and Nitrogen Compounds (SK025)

Properties and Combustion of Alkanes

Alkanes, such as butane, exhibit specific physical properties relating to their solubility. Butane is a non-polar molecule and is unable to form hydrogen bonds with water. Consequently, butane is insoluble in water because it cannot interact through hydrogen bonding with polar water molecules. However, butane is soluble in non-polar solvents like benzene. This solubility occurs because butane, being non-polar, is able to form van der Waals forces with benzene molecules.

The chemical behavior of alkanes is characterized significantly by combustion. Alkanes are burnt in air or oxygen to produce carbon dioxide gas, water, and heat energy. There are three primary types of combustion based on the availability of oxygen:

1. Combustion in excess oxygen (complete combustion): The alkane reacts fully with oxygen to produce carbon dioxide and water. For butane, the equation is: $C_4H_{10} + \frac{13}{2}O_2 \rightarrow 4CO_2 + 5H_2O + \text{heat}$

2. Combustion in limited oxygen (incomplete combustion): Not all carbon is converted to carbon dioxide; some forms carbon monoxide ($CO$). For butane: $C_4H_{10} + \frac{9}{2}O_2 \rightarrow 4CO + 5H_2O + \text{heat}$

3. Combustion in very limited oxygen (incomplete combustion): The carbon in the hydrocarbon is converted to elemental carbon, also known as soot. For butane: $C_4H_{10} + \frac{5}{2}O_2 \rightarrow 4C + 5H_2O + \text{heat}$

Halogenation of Alkanes (Free Radical Substitution)

Alkanes react with halogens such as chlorine ($Cl_2$) and bromine ($Br_2$) to produce haloalkanes. This reaction requires the presence of light ($uv$ or $hv$) or high temperatures greater than $100\,^{\circ}\text{C}$. The specific function of sunlight in this process is to provide the energy necessary for homolytic cleavage of the halogen-halogen bond. The general reaction is represented as: $R-H + X_2 \xrightarrow{hv} R-X + HX$

In these reactions, the type of hydrogen atom involved is crucial. Equivalent types of hydrogen atoms are identical in their chemical environment. For example, in methane ($CH_4$), all four hydrogens are equivalent: $H-C(H)_3 + Br_2 \xrightarrow{uv} CH_3Br + HBr$

Another example is neopentane (2,2-dimethylpropane), where all twelve hydrogens are equivalent, reacting with chlorine to form a single chloro-product: $(CH_3)_3CCH_3 + Cl_2 \xrightarrow{uv} (CH_3)_3CCH_2Cl + HCl$

Non-equivalent types of hydrogen atoms exist when hydrogens occupy different chemical environments within the same molecule, leading to multiple possible haloalkane products.

Preparation of Alkenes through Elimination

Alkenes can be synthesized through elimination reactions, primarily via the dehydration of alcohols or the dehydrohalogenation of haloalkanes.

Dehydration of alcohols involves the elimination of a water ($H_2O$) molecule. This process requires concentrated sulfuric acid ($H_2SO_4$) or phosphoric acid ($H_3PO_4$), which act as acidic catalysts and dehydrating agents. The general equation is: $-C(H)-C(OH)- \xrightarrow{\text{conc. } H_2SO_4, \Delta} C=C + H_2O$

Dehydrohalogenation of haloalkanes involves the elimination of a hydrogen atom and a halogen atom from an alkyl halide to form an alkene. This requires a strong base such as $OH^-$ or $OR^-$ in an alcohol solvent (e.g., ethanol) under reflux. The general equation is: $-C(H)-C(X)- + KOH \xrightarrow{\text{ethanol, reflux}} C=C + KX + H_2O$

To determine the major product in these elimination reactions, Saytzeff's rule is applied. Saytzeff's rule states that the most stable alkene product is the one that is the most highly substituted (the one with the most alkyl groups attached to the double-bonded carbons).

Electrophilic Addition Reactions of Alkenes

Alkenes undergo electrophilic addition due to the high electron density of the carbon-carbon double bond. Several specific reactions are detailed:

1. Hydrogenation: Addition of hydrogen ($H_2$) in the presence of a catalyst like Platinum ($Pt$). For example, propene reacts with $H_2$ and $Pt$ to form propane.

2. Halogenation in Inert Solvent: Addition of $Cl_2$ or $Br_2$ in an inert solvent like dichloromethane ($CH_2Cl_2$). This results in a vicinal dihalide ($R-CH(X)-CH_2X$).

3. Halogenation in Water (Halohydrin Formation): When $Cl_2$ or $Br_2$ is added in the presence of water, the halogen follows Markovnikov's rule, and a halohydrin is formed ($R-CH(OH)-CH_2X$).

4. Hydrogen Halides: Addition of $HCl$ or $HBr$. The hydrogen atom follows Markovnikov's rule, attaching to the carbon with more hydrogen atoms.

5. Acidified Water (Hydration): Addition of $H_2O$ catalyzed by an acid ($H^+$). Hydrogen follows Markovnikov's rule to produce an alcohol.

6. Addition of $HBr$ in Peroxides: When peroxides ($ROOR$) are present, the addition of $HBr$ follows the anti-Markovnikov rule, where the hydrogen attaches to the carbon with fewer hydrogen atoms.

Oxidation Reactions of Alkenes

Alkenes undergo oxidation where the $C=C$ bond is cleaved or transformed.

Ozonolysis involves reacting an alkene with ozone ($O_3$), followed by a reductive workup using Zinc ($Zn$) and water ($H_2O$) or dimethyl sulfide ($(CH_3)_2S$). This process causes oxidative cleavage at the double bond, yielding ketones or aldehydes. For example, an alkene with a substituted double bond will break into two carbonyl fragments.

Reaction with hot, acidified potassium permanganate ($KMnO_4, H^+, \Delta$) also results in oxidative cleavage. Depending on the substitution of the double bond:

• A terminal $=CH_2$ group is oxidized to $CO_2$ and $H_2O$.
• A monosubstituted carbon ($=CH-R$) is oxidized to a carboxylic acid ($R-COOH$).
• A disubstituted carbon ($=C(R)_2$) is oxidized to a ketone ($R-C(=O)-R$).

Unsaturation Tests for Alkenes

Three primary tests are used to identify the presence of unsaturation (double bonds) in alkenes:

1. Baeyer's Test: Uses dilute alkaline $KMnO_4$ solution in the cold. The observation is that the purple color of the $KMnO_4$ solution is decolourised, and a brown precipitate of manganese dioxide ($MnO_2$) is formed. This produces a vicinal diol: $H_2C=CH_2 + KMnO_4 \xrightarrow{OH^-, H_2O, \text{cold}} HO-CH_2-CH_2-OH + MnO_2(s)$

2. Bromine in Dichloromethane ($CH_2Cl_2$): The reddish-brown color of bromine is decolourised as it adds across the double bond to form a colorless haloalkane.

3. Bromine Water: The reddish-brown color of bromine is decolourised. This reaction follows Markovnikov's rule to form a halohydrin (an alcohol with a halogen on the adjacent carbon).

Polymerization of Alkenes

Alkenes serve as monomers that can link together to form long-chain polymers in the presence of a peroxide catalyst like $H_2O_2$.

1. Polyethylene: Produced from the monomer ethene ($CH_2=CH_2$). The double bond breaks to form a repeating unit $-[CH_2-CH_2]_n-$.
2. Polystyrene: Produced from phenylethene (styrene, $CH_2=CH(C_6H_5)$). The repeating unit is $-[CH_2-CH(C_6H_5)]_n-$.
3. Polyvinyl Chloride (PVC): Produced from chloroethene (vinyl chloride, $CH_2=CHCl$). The repeating unit is $-[CH_2-CHCl]_n-$.
4. Teflon (Polytetrafluoroethene): Produced from tetrafluoroethene ($CF_2=CF_2$). The repeating unit is $-[CF_2-CF_2]_n-$.

Electrophilic Aromatic Substitution (EAS) of Benzene

Benzene undergoes substitution reactions where an electrophile ($E^+$) replaces a hydrogen atom on the ring. There are four major types of EAS:

1. Nitration: Reagent is concentrated nitric acid ($HNO_3$) with concentrated sulfuric acid ($H_2SO_4$) as a catalyst at $50-55\,^{\circ}\text{C}$. A nitro group ($-NO_2$) replaces a hydrogen.
2. Halogenation: Reagents are halogens ($Cl_2$ or $Br_2$) with Lewis acid catalysts such as $FeCl_3$ or $FeBr_3$. A halogen atom ($Cl$ or $Br$) replaces a hydrogen.
3. Friedel-Crafts Alkylation: Reagent is a haloalkane ($RX$) with a Lewis acid catalyst ($AlCl_3$ or $AlBr_3$). An alkyl group ($R$) replaces a hydrogen.
4. Friedel-Crafts Acylation: Reagent is an acid chloride ($RCOCl$) with an $AlCl_3$ catalyst. An acylium group ($R-C=O$) replaces a hydrogen.

Lewis acids act as catalysts by polarizing the halogen, haloalkane, or acid chloride molecule to generate the electrophile.

Reactions of Alkyl Benzenes

Alkyl benzenes exhibit reactivity both on the ring and the side chain:

1. Oxidation: Benzene itself does not react with hot acidified $KMnO_4$ or $K_2Cr_2O_7$. However, the benzylic hydrogen of an alkylbenzene (the hydrogen on the carbon directly attached to the ring) can be oxidized to a carboxyl group ($-COOH$), forming benzoic acid. This requires at least one benzylic hydrogen.
2. Halogenation: In the presence of high temperature or UV light, a hydrogen atom in the alkyl group is substituted by a halogen via a free radical substitution mechanism, rather than ring substitution.

Reactions and Properties of Haloalkanes

Haloalkanes undergo two major types of reactions: Nucleophilic Substitution ($S_N$) and Elimination ($E$).

Nucleophilic Substitution involve a nucleophile ($Nu^-$) replacing the halogen leaving group ($X^-$). Key reactions include:

• Forming Alcohols: Using $NaOH(aq)$ or $H_2O$ under reflux.
• Forming Amines: Using excess ammonia ($NH_3$) under heat.
• Forming Nitriles: Using $KCN$ in alcohol under reflux.
• Forming Ethers: Using an alcohol ($R'OH$) or a sodium alkoxide ($R'ONa$).
• Forming Esters: Using a sodium carboxylate ($R'COONa$).

Elimination (Dehydrohalogenation) occurs when haloalkanes are heated under reflux with alcoholic strong bases (e.g., $NaOH$, $KOH$, $NaOCH_3$, $NaOCH_2CH_3$, or $KOC(CH_3)_3$). This removes a halogen and a hydrogen from adjacent carbons to form an alkene. Saytzeff's rule identifies the major product as the most substituted alkene.

Grignard Reagents

Grignard reagents are organomagnesium halides with the empirical formula $RMgX$ or $ArMgX$. They are prepared by reacting a haloalkane with magnesium metal in anhydrous (dry) ether.

Grignard reagents act as powerful nucleophiles in synthesizing various organic compounds:

1. Carboxylic Acids: React with carbon dioxide ($CO_2$) followed by hydrolysis ($H_3O^+$).
2. Primary (1°) Alcohols: React with methanal ($H_2C=O$) followed by hydrolysis.
3. Secondary (2°) Alcohols: React with other aldehydes ($RCHO$) followed by hydrolysis.
4. Tertiary (3°) Alcohols: React with ketones ($R_2C=O$) followed by hydrolysis.

Preparation and Properties of Alcohols

Alcohols can be prepared through several methods:

• Fermentation of carbohydrates using yeast: $C_6H_{12}O_6 \rightarrow 2CH_3CH_2OH + 2CO_2$.
• Hydration of alkenes using $H_2O$ and $H_2SO_4$ (Markovnikov's rule).
• Hydrolysis of haloalkanes using a strong base like $NaOH(aq)$ under reflux.
• Addition of Grignard reagents to carbonyl compounds.

Chemical properties include:

• Acidity: Alcohols act as weak acids, reacting with sodium metal to donate a hydrogen ion ($H^+$) and form a sodium alkoxide ($RONa$). Phenol is more acidic than aliphatic alcohols and reacts with $NaOH$, whereas aliphatic alcohols do not.
• Esterification: Reaction with carboxylic acids using concentrated $H_2SO_4$ and heat to form esters.
• Dehydration: Elimination of water to form alkenes (Saytzeff's rule applies).
• Substitution: Using $HX$, $PX_3$, $PCl_5$, or $SOCl_2$ to replace the hydroxyl group with a halogen.

Oxidation and Identification of Alcohols

Oxidation depends on the class of alcohol and the strength of the oxidizing agent:

• Primary Alcohols: Oxidize to aldehydes using a weak agent (PCC in $CH_2Cl_2$) or to carboxylic acids using strong agents ($KMnO_4$ or $K_2Cr_2O_7, H^+, \Delta$).
• Secondary Alcohols: Oxidize to ketones using either weak or strong agents.
• Tertiary Alcohols: Resist oxidation with both weak and strong agents.

Identification Tests:

1. Lucas Test: Uses concentrated $HCl$ and $ZnCl_2$. Tertiary alcohols turn the solution cloudy immediately; secondary alcohols turn cloudy within 5 minutes; primary alcohols show no reaction within 10 minutes.
2. Iodoform Test: Identifies the methyl carbinol group ($CH_3CH(OH)-$). Reaction with $I_2$ in $OH^-$ produces a yellow precipitate of iodoform ($CHI_3$).

Properties and Tests of Phenol

Phenol reacts with Group 1 metals (Na) and bases (NaOH). In the reaction with $NaOH$, a single-layer solution forms (sodium phenoxide). This differs from aliphatic alcohols which are insoluble in $NaOH$ (forming two layers).

Chemical Tests for Phenol:

1. Ferric Chloride ($FeCl_3$): The yellow color of aqueous $FeCl_3$ turns purple in the presence of phenol.
2. Bromine Water: Reaction with aqueous bromine produces a white precipitate (2,4,6-tribromophenol).

Preparation and Reactions of Carbonyl Compounds

Carbonyls (aldehydes and ketones) are prepared via:

• Ozonolysis of alkenes.
• Friedel-Crafts acylation (for aromatic ketones).
• Oxidation of primary alcohols (to aldehydes) or secondary alcohols (to ketones).

Chemical Properties of Aldehydes and Ketones:

• Nucleophilic Addition: Addition of $KCN$, $H_2O$, alcohols ($CH_3OH$), $NaHSO_3$, or Grignard reagents to the $C=O$ group.
• Reduction: Carbonyls are reduced to alcohols using $LiAlH_4$ followed by $H_2O$, $NaBH_4$ followed by $H_3O^+$, or $H_2$ with a $Pt$ catalyst.
• Condensation: Reactions with ammonia derivatives ($NH_3$, $NH_2OH$, $N_2H_4$, or 2,4-DNPH) involve the removal of a water molecule to form imines or substituted variants.
• Oxidation: Aldehydes are easily oxidized to carboxylic acids ($KMnO_4$ or $K_2Cr_2O_7$), while ketones are resistant to oxidation.

Identification Tests for Carbonyls

1. Brady's Test (2,4-DNPH): Identifies the presence of a carbonyl group ($C=O$). Both aldehydes and ketones form an orange precipitate.
2. Tollens' Test: Distinguishes aldehydes from ketones. Aldehydes react with the diaminosilver ion ($[Ag(NH_3)_2]^+$) to form a silver mirror. Ketones show no reaction.
3. Fehling's Test and Benedict's Test: Distinguish aldehydes from ketones. Aldehydes react with $Cu^{2+}$ in alkaline conditions to form a brick red precipitate ($Cu_2O$). Ketones show no reaction.
4. Iodoform Test: Specifically identifies methyl ketones ($R-C(=O)CH_3$). Reaction with $I_2/NaOH$ yields a yellow precipitate of $CHI_3$.

Carboxylic Acids and Amines

Carboxylic acids are prepared through:

• Oxidation of primary alcohols or aldehydes.
• Hydrolysis of nitriles ($RCN$).
• Carbonation of Grignard reagents ($RMgX + CO_2$ followed by hydrolysis). Reactions include neutralization with bases (forming salts and $CO_2$ with carbonates), reduction to alcohols ($LiAlH_4$), formation of acyl chlorides ($SOCl_2$, $PCl_3$, $PCl_5$), esterification, and formation of amides or anhydrides.

Amines are categorized as primary (1°), secondary (2°), or tertiary (3°).

• Preparation: Reduction of nitro compounds ($Sn/HCl$), nitriles ($LiAlH_4$ or $H_2/catalyst$), or amides ($LiAlH_4$). Hoffmann’s degradation of primary amides using $X_2$ and $NaOH$ yields an amine with one fewer carbon atom.
• Hinsberg's Test: Distinguishes classes using benzene sulphonyl chloride. Primary amines form a precipitate that dissolves in excess $KOH$. Secondary amines form a precipitate that remains. Tertiary amines form two layers but no reaction; they dissolve in $HCl$.
• Nitrous Acid Test ($HNO_2$): Primary aliphatic amines release $N_2$ gas (effervescence). Primary aromatic amines form stable diazonium salts at $0-5\,^{\circ}\text{C}$. Secondary amines form yellow oil. Tertiary aliphatic amines form clear solutions, while aromatic ones can form green precipitates.