Chemistry · Organic chemistry · Grade 10

Isomers and nomenclature

Butane and isobutane in a lighter have the same formula — so why are they different substances?

The situation

In the transparent body of a lighter a liquid sloshes — this is butane C₄H₁₀, liquefied under pressure. But the gas sold as “butane” is often a mixture of two substances: ordinary butane boils at −0.5 °C and isobutane at −11.7 °C. Both have 4 carbons and 10 hydrogens — the same formula. How can one formula fit two different substances, how do we name them, and into what classes are organic substances divided?

Topic

Isomerism, classification and nomenclature. Lighter gas: butane and isobutane — one formula, different structure and boiling point; ball-and-stick models

Key ideas

Structural and spatial isomerism
IUPAC nomenclature
Classes of organic compounds

Formulas

C_{4} H_{10} : C H_{3} - C H_{2} - C H_{2} - C H_{3} and C H_{3} - C H (C H_{3}) - C H_{3}

Chain isomers: butane (−0.5 °C) and 2-methylpropane, that is isobutane (−11.7 °C).

C_{n} H_{2 n + 2} : n = 4, 5, 6, 7, 8, 10 \Rightarrow 2, 3, 5, 9, 18, 75 isomers

As the number of carbon atoms grows, the number of structural isomers rises quickly.

CH_{4} + Cl_{2} \overset{\overset{}{hν}}{\to} CH_{3} Cl + HCl C_{2} H_{4} + Br_{2} \overset{}{\to} C_{2} H_{4} Br_{2}

Substitution and addition reactions.

3-ethyl-2-methylpentane : \underset{alphabetical}{\underset{⏟}{3 -ethyl}} - \underset{}{\underset{⏟}{2 -methyl}} \underset{5 C}{\underset{⏟}{pent}} \underset{C–C}{\underset{⏟}{ane}}

IUPAC name: substituents (with numbers, in alphabetical order) + root of the main chain + suffix.

Theory

Isomerism and its types (1.4)

Isomers are substances with the same composition (molecular formula) but different structure and therefore different properties. This is a direct consequence of Butlerov's theory. Structural isomerism: *carbon skeleton* (chain) isomerism — butane and isobutane; *functional group (or double bond) position* isomerism — 1-chloropropane and 2-chloropropane, propan-1-ol and propan-2-ol, but-1-ene and but-2-ene; *interclass* isomerism — ethanol (an alcohol) and dimethyl ether (an ether). Spatial isomerism (stereoisomerism): the order of atoms is the same, but their arrangement in space differs. *Geometric (cis–trans)* isomerism occurs about a double bond, because there is no rotation about C=C: cis-but-2-ene (3.7 °C) and trans-but-2-ene (0.9 °C) are different substances. (Optical isomerism is studied in the higher grades.)

Classification of organic compounds (1.5)

By carbon skeleton: acyclic (open-chain) and cyclic compounds. Hydrocarbons consist only of C and H: saturated — alkanes CₙH₂ₙ₊₂ and cycloalkanes CₙH₂ₙ; unsaturated — alkenes CₙH₂ₙ (C=C), alkadienes CₙH₂ₙ₋₂ (two C=C), alkynes CₙH₂ₙ₋₂ (C≡C); aromatic — arenes CₙH₂ₙ₋₆ (benzene ring). The other classes form when a functional group takes the place of hydrogen in a hydrocarbon molecule, and this group determines the properties of the class: halogen derivatives (–Hal), alcohols and phenols (–OH), ethers (–O–), aldehydes (–CHO), ketones (>C=O), carboxylic acids (–COOH), esters (–COO–), amines (–NH₂), nitro compounds (–NO₂).

Types of reaction (1.6) and models (2.21)

Substitution: one atom or group is replaced by another — $CH_{4} + Cl_{2} \overset{\overset{}{hν}}{\to} CH_{3} Cl + HCl$ (typical of alkanes). Addition: a multiple bond breaks and two molecules become one — $C_{2} H_{4} + Br_{2} \overset{}{\to} C_{2} H_{4} Br_{2}$ , and bromine water is decolourised. Elimination: a small molecule is split off from a molecule — water is eliminated from ethanol to give ethene (conc. H₂SO₄, ≈ 170 °C). Isomerisation: the composition does not change, the structure does — butane turns into isobutane in the presence of AlCl₃. Models: the *ball-and-stick* model shows the order of the atoms, the bond angles (109.5°, 120°, 180°) and lengths; the *space-filling* (Stuart–Briegleb) model shows the true relative sizes of the atoms (van der Waals radii) and the shape of the molecule. In the practical lesson, models of methane, ethane, butane and isobutane, ethene and ethyne (acetylene) are built.

Nomenclature (1.7–1.8): IUPAC rules

1. Find the longest carbon chain — the main chain (if two are equal, the one with more substituents). 2. Number the main chain from the end nearest a substituent, so that the set of numbers is the smallest (the first point of difference should be the smaller number). 3. Write the substituents (alkyl radicals: methyl CH₃–, ethyl C₂H₅–, propyl, isopropyl) with their numbers; identical ones are combined with “di-”, “tri-”, “tetra-”, and each is given a number: 2,2-dimethyl. 4. Different substituents go in alphabetical order (ethyl, then methyl); commas between numbers, hyphens between a number and a word. 5. At the end the name of the main chain: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec- + “-ane”. For example, CH₃–CH(CH₃)–CH₂–C(CH₃)₂–CH₃ is 2,2,4-trimethylpentane (isooctane). Carbon atoms can be primary (bonded to one C), secondary, tertiary and quaternary. Trivial (older) names are also met: isobutane, isopentane, neopentane.

Why do the properties of isomers differ?

Alkane molecules are attracted to one another only by weak van der Waals forces; these depend on the contact surface of the molecules. Long “zigzag” butane molecules approach one another along a wide surface, while spherical isobutane and neopentane touch only at “points” — so the branched isomer boils at a lower temperature: pentane 36.1 °C, 2-methylbutane 27.8 °C, 2,2-dimethylpropane 9.5 °C. For interclass isomers the difference is even bigger: ethanol molecules form hydrogen bonds with one another (78.3 °C), whereas dimethyl ether has no O–H (−24.8 °C).

About the model

The models are built with real bond lengths (C–C 154 pm, C=C 134 pm, C–H 109 pm) and angles, magnified ≈ 5·10⁸ times; the conformation is the most favourable “zigzag”. Boiling points are at 101.3 kPa (CRC Handbook). The reaction animations are schematic: molecules approach, the atoms that change light up, then the products appear — a real mechanism (for example a radical chain reaction) proceeds step by step. The naming rules for alkanes are checked in the program (IUPAC, Uzbek form).

Did you know?

In the octane number, which measures the resistance of petrol to knocking, 2,2,4-trimethylpentane (isooctane) is taken as 100, and normal heptane, which is not an isomer of it, as 0.
The number of structural isomers of alkanes grows quickly as carbons are added: C₁₀H₂₂ has 75, and C₂₀H₄₂ has 366 319.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Chemistry in our body — Grade 7
Organic chemistry: structure — Grade 10
Alkanes and cycloalkanes — Grade 10
Alkenes and alkadienes — Grade 10
Polymers, rubber and fibres — Grade 10
Alkynes, benzene and styrene — Grade 10
Natural gas, oil and coal — Grade 10
Alcohols and ethers — Grade 10
Phenol, aldehydes and ketones — Grade 10
Acids, esters and soap — Grade 10