Alkanes: Structure, Nomenclature and Physical Properties: quick revision, 15 bites

Alkanes Are Saturated — That's Their Whole Identity

Single bonds only. No exceptions.

Alkanes are **saturated hydrocarbons** — they contain only carbon-carbon single bonds and C-H bonds, with no double or triple bonds anywhere.

Takeaway. If a molecule has any double or triple bond, it is NOT an alkane.

Why Alkanes Are Called 'Paraffins'

Latin for 'little affinity' — and it fits.

Alkanes are highly **inert** toward common reagents like $KMnO_4$, sodium dichromate, nitric acid, and hydrogen peroxide. This lack of reactivity earned them the name **paraffins**.

Takeaway. Alkane inertness is a direct consequence of strong, stable sigma bonds.

sp³ Hybridization Builds the Tetrahedral Frame

Every carbon sits at a tetrahedron's center.

In alkanes, each carbon is **sp³ hybridized**, giving a tetrahedral geometry with bond angles of about $109.5^\circ$.

Takeaway. sp³ hybridization explains both the shape and the stability of alkanes.

The General Formula for Alkanes

One formula, every alkane.

This formula defines the **homologous series** of alkanes — same functional behavior, different chain lengths.

Takeaway. Plug in n to get the molecular formula of any alkane.

Root Words: Name Follows Carbon Count

Count carbons, pick the root.

Takeaway. The root word is fixed by the number of carbons in the parent chain.

Primary Suffixes Reveal the Bond Type

One suffix tells you the bond.

For alkanes, the primary suffix is always **-ane**.

Takeaway. Root + primary suffix = IUPAC name of the alkane.

Naming a Branched Alkane: Step-by-Step

Longest chain first. Always.

Takeaway. Longest chain → lowest locants → alphabetical order. That's the IUPAC sequence.

Worked Example: 2,3,5-Trimethylhexane

Left gives 11. Right gives 11. Now what?

An 8-carbon chain with methyl groups at positions 2, 3, 5, 6 from the left gives sum = **16**; from the right gives **11**. So numbering starts from the right.

Takeaway. Lowest sum of locants decides the numbering direction.

Don't Number From the Wrong End

Lowest first substituent beats lowest sum? Not always.

Takeaway. One substituent → lowest single locant. Multiple → lowest sum.

Straight-Chain vs Branched Alkanes

Same formula, different boiling points.

Takeaway. Branching reduces surface contact → weaker intermolecular forces → lower boiling point.

Physical State Follows Chain Length

Gas → liquid → solid as carbons grow.

Takeaway. All alkanes are colorless and odorless — only the physical state changes with size.

Solubility: Nonpolar Means Water-Shy

Like dissolves like — alkanes avoid water.

Alkanes are **nonpolar**, so they do not dissolve in polar solvents like water but dissolve readily in nonpolar solvents such as benzene, ether, and carbon tetrachloride.

Takeaway. Nonpolar nature explains both insolubility in water and solubility in organic solvents.

Boiling Point Climbs ~20–30 °C Per CH₂

Add one CH₂, raise the boil by 20–30 °C.

Takeaway. Boiling point is regular; melting point is not — don't confuse the two trends.

Even-Carbon Alkanes Melt Higher

Even carbons pack better — and melt higher.

Alkanes with an **even number of carbon atoms** have higher melting points than their odd-numbered neighbors because they pack more symmetrically, allowing greater intermolecular attraction.

Takeaway. Even-carbon alkanes pack better → higher melting point.

Exam Favorite: Why Are Alkanes Inert?

This question appears every year.

Takeaway. Three keywords — sp³, sigma bonds, nonpolar — earn full marks.