STPM Chemistry Semester 3: Chapter 2 - Hydrocarbon

Comprehensive practice flashcards for STPM Chemistry Semester 3 Chapter 2: Hydrocarbon, covering nomenclature, physical properties, structural and stereoisomerism, reaction mechanisms, cracking, pollution control, and alkene addition/oxidation reactions.

What is the chemical definition of a hydrocarbon?

Hydrocarbons are compounds composed entirely of carbon and hydrogen.

Why are hydrocarbons generally hydrophobic?

Nearly all hydrocarbons are nonpolar or only slightly polar, which prevents them from forming hydrogen bonds with water and causes them to be hydrophobic.

How are saturated hydrocarbons distinguished structurally from unsaturated hydrocarbons?

Saturated hydrocarbons contain only $C-C$ single bonds in their carbon chains (such as alkanes or cycloalkanes), whereas unsaturated hydrocarbons contain at least one $\pi$-bond ($C=C$ or $C\equiv C$) in their carbon chains (such as alkenes, alkynes, or aromatic compounds).

What is the general molecular formula and type of covalent bonding characteristic of alkanes?

Alkanes have the general formula $C_n H_{2n+2}$ and contain only $\sigma$-bonds between carbon atoms.

What is an alkyl group, and how does it differ from an alkane?

An alkyl group is an alkane lacking one hydrogen atom; it acts as a substituent or branch attached to a parent organic compound.

What is the general molecular formula of a cycloalkane, and what is the minimum number of carbon atoms required to form one?

The general formula is $C_n H_{2n}$, and a minimum of $3$ carbon atoms is required to form a cyclic structure.

What are the three main subcategories of structural (constitutional) isomerism?

The three subcategories are chain isomerism, positional isomerism, and functional isomerism.

Which classes of organic compounds form functional isomer pairs sharing the same general molecular formulas?

1. Alkene and cycloalkane ($C_n H_{2n}$)
2. Alcohol and ether ($C_n H_{2n+2}O$)
3. Aldehyde and ketone ($C_n H_{2n}O$)
4. Carboxylic acid and ester ($C_n H_{2n}O_2$)

What two structural conditions are required for a molecule to exhibit geometrical (cis-trans) isomerism?

1. The presence of a restricted rotation system, either a $C=C$ double bond or a saturated cyclic structure.
2. Each carbon atom involved in the restricted rotation must be bonded to two different atoms or groups of atoms.

What structural feature is required for an organic molecule to exhibit optical isomerism?

The molecule must have a chiral carbon atom that is $sp^3$-hybridised and bonded to four different atoms or groups of atoms, producing non-superimposable mirror-image enantiomers.

What is the minimum number of carbon atoms required for an alkane to exhibit optical isomerism?

An alkane must contain at least $7$ carbon atoms (such as 3-methylhexane or 2,3-dimethylpentane).

Why does the boiling point of unbranched alkanes increase down the homologous series?

Alkanes are simple covalent molecules held by weak Van der Waals forces. As molecular mass and size increase with carbon number, the strength of the Van der Waals forces increases, requiring more heat energy to overcome.

Why do straight-chain alkanes have higher boiling points than their branched isomers of the same molecular formula?

Straight-chain alkanes possess a larger total surface area than branched isomers, resulting in stronger intermolecular Van der Waals forces.

What reagents and conditions are required for the preparation of an alkane via hydrogenation of an alkene or alkyne?

Hydrogen gas ($H_2$) with a Nickel ($Ni$) catalyst heated at $180\,^\circ\text{C}$, or a Platinum ($Pt$) catalyst at room condition.

How does ethane behave when treated with aqueous sodium hydroxide, concentrated hydrochloric acid, or acidified potassium manganate(VII)?

Ethane has no reaction (no effect) under either hot or cold conditions because it is an unreactive, nonpolar, saturated hydrocarbon.

What are the three elementary steps in the free radical substitution mechanism of an alkane?

1. Initiation (formation of halogen radicals via homolytic fission under ultraviolet light)
2. Propagation (chain reactions propagating alkyl and halogen radicals)
3. Termination (combination of two radicals to form a stable covalent molecule)

How can the formation of polyhalogenated side products be minimised during alkane halogenation?

By reacting the halogen with an excess of the alkane, which decreases the probability of halogen radicals reacting with already halogenated intermediates.

What is the order of relative stability among carbon radicals, and what explains this trend?

The stability increases in the order: methyl radical $<$ primary ($1^\circ$) $<$ secondary ($2^\circ$) $<$ tertiary ($3^\circ$). This is due to greater hyperconjugation stabilizing the radical as the number of surrounding alkyl groups increases.

What is the difference between thermal cracking (pyrolysis) and catalytic cracking of hydrocarbons?

Thermal cracking uses high temperature and homolytic fission under anaerobic conditions to produce unbranched alkanes and alkenes. Catalytic cracking uses zeolite or aluminium oxide catalysts at lower temperatures to produce more branched alkanes and alkenes.

What role does branching play in determining the Relative Octane Number (RON) of an octane chain?

A greater number of branches in the octane chain leads to a higher Relative Octane Number (RON).

What chemical pollutants from vehicle exhaust are mitigated by catalytic converters, and into what products are they converted?

Catalytic converters oxidise poisonous carbon monoxide ($CO$) and unburned hydrocarbons ($C_x H_y$) into carbon dioxide ($CO_2$) and water ($H_2O$), and reduce hazardous nitrogen oxides ($NO_x$) to nitrogen gas ($N_2$) and oxygen gas ($O_2$).

Why were alkenes historically referred to as 'olefins' or 'olefiant gas'?

Ethene was called olefiant gas (from Latin oleum, oil + facere, to make) because gaseous ethene ($C_2H_4$) reacts with chlorine to produce the oily liquid 1,2-dichloroethane ($C_2H_4Cl_2$).

What is the minimum number of carbon atoms required for an alkene to exhibit optical isomerism?

An alkene must have at least $6$ carbon atoms (such as 3-methylpent-1-ene).

Why do trans-alkenes have higher melting points but lower boiling points than their cis-isomers?

Trans-isomers have higher melting points because their more symmetrical shape allows closer, more effective crystal packing in the solid state. They have lower boiling points because their opposing bond dipole vectors cancel out, whereas cis-isomers have a net dipole moment making them more polar.

How does alkyl substitution affect the relative thermodynamic stability of alkenes?

Alkene stability increases as the degree of alkyl substitution on the double bond increases: unsubstituted $<$ monosubstituted $<$ disubstituted $<$ trisubstituted $<$ tetrasubstituted.

What are the reagents and conditions for the dehydrohalogenation of a haloalkane and the dehydration of an alcohol to prepare an alkene?

Dehydrohalogenation requires ethanolic potassium hydroxide ($KOH$) under reflux/heat ($\Delta$). Dehydration requires excess concentrated sulphuric acid ($H_2SO_4$) heated at $180\,^\circ\text{C}$.

What does Zaitsev's rule state regarding elimination reactions?

Elimination of a hydrogen halide or water preferentially forms the alkene with more alkyl substituents (hydrogen is preferentially removed from the adjacent carbon with fewer hydrogen atoms).

What does Markovnikov's rule state for the addition of hydrogen halides ($HX$) to unsymmetrical alkenes?

When an unsymmetrical alkene reacts with a hydrogen halide, the halide nucleophile attaches to the carbon atom with more alkyl substituents, while the hydrogen electrophile adds to the carbon atom bearing more hydrogen atoms.

What is the order of carbocation stability, and why do alkyl groups stabilize carbocations?

Stability increases in the order: primary ($1^\circ$) $<$ secondary ($2^\circ$) $<$ tertiary ($3^\circ$). Alkyl groups act as electron-donating groups that push electron density toward the positively charged carbon ($C^+$), dispersing the positive charge.

What is the relative order of reactivity for hydrogen halides in electrophilic addition to alkenes?

Reactivity increases in the order: $HF < HCl < HBr < HI$, which is governed by decreasing bond strength and increasing bond length.

What cyclic intermediate forms during the electrophilic addition of halogens ($X_2$) to alkenes in an organic solvent, and what stereochemical product results?

A cyclic bridged halonium ion (such as a bromonium ion) forms in the first step. The halide ion then attacks from the opposite face to avoid steric hindrance, resulting in anti-addition and the formation of a trans-dihaloalkane.

What is the visual observation when an alkene is tested with bromine dissolved in dichloromethane or bromine water?

The reddish-brown colour of bromine is decolourised.

What is a halohydrin, and why is it the major product when an alkene reacts with halogen in aqueous solution ($X_2(aq)$)?

A halohydrin is an alcohol with a halogen atom on the adjacent carbon atom. It is the major product because polar water molecules are present in large excess over halide ions to act as the nucleophile attacking the halonium ion intermediate.

What reagents and conditions are used in the large-scale industrial preparation of ethanol from ethene?

Reacting ethene with steam ($H_2O(g)$) catalysed by phosphoric acid ($H_3PO_4(aq)$) at $300\,^\circ\text{C}$.

What type of carbocation rearrangement can occur during the acid-catalysed hydration of alkenes?

A 1,2-hydride shift or 1,2-methyl shift occurs when an initially formed secondary ($2^\circ$) carbocation can rearrange into a more stable tertiary ($3^\circ$) carbocation.

What are the observation and products when an alkene reacts with cold, dilute, alkaline potassium manganate(VII)?

A diol (glycol) is formed, and the purple colour of the $KMnO_4$ solution is decolourised to form an opaque brown precipitate of manganese(IV) oxide ($MnO_2$).

What cleavage products are produced when hot, concentrated, acidified $KMnO_4$ oxidises unsubstituted ($=CH_2$), monosubstituted ($=CHR$), and disubstituted ($=CR_2$) alkene carbons?

• Unsubstituted carbon ($=CH_2$) oxidises to $CO_2 + H_2O$.
• Monosubstituted carbon ($=CHR$) oxidises to a carboxylic acid ($RCOOH$).
• Disubstituted carbon ($=CR_2$) oxidises to a ketone ($RC(=O)R$).

How can hot, concentrated, acidified $KMnO_4$ be used to distinguish an alk-1-ene from other alkene isomers?

While both decolourise the purple colour of $KMnO_4$, the terminal alk-1-ene produces carbon dioxide gas from its $=CH_2$ group, causing visible bubbling/effervescence, whereas internal alkenes do not.

What reagents are used for the ozonolysis of alkenes, and what is its synthetic advantage over hot acidified $KMnO_4$?

Ozone in dichloromethane ($O_3$ in $CH_2Cl_2$) at $-78\,^\circ\text{C}$ followed by dimethyl sulphide ($DMS$) or $Zn$ in $CH_3COOH$. It cleaves $C=C$ bonds strictly to carbonyl compounds (aldehydes and ketones) without further oxidising aldehydes into carboxylic acids.

How do the catalysts and resulting polymer structures differ between High Density Polyalkene (HDPE) and Low Density Polyalkene (LDPE)?

HDPE is produced using a Ziegler-Natta catalyst ($TiCl_4$ with $R_3Al$) via anionic addition to yield linear, orderly (syndiotactic) polymers of higher density. LDPE is produced using benzoyl peroxide via free radical addition to yield branched, random (atactic) polymers of lower density.