Alkane Representations

Molecular Formula

A formula that shows the number and types of atoms present in a molecule but does not show how those atoms are connected.

Structural Formula

A representation that shows how the atoms in a molecule are connected to one another.

Expanded Structural Formula

A structural representation that explicitly shows the individual atoms and bonds in a molecule.

Condensed Structural Formula

A compact structural representation in which many or all individual C–H bonds are omitted and atoms are grouped according to their connectivity.

Condensed Formula of Butane

CH₃CH₂CH₂CH₃

Alternative Condensed Formula of Butane

CH₃(CH₂)₂CH₃

Meaning of (CH₂)₂ in CH₃(CH₂)₂CH₃

The CH₂ group occurs twice consecutively in the carbon chain.

Molecular Formula of Butane

C₄H₁₀

Does C₄H₁₀ alone specify the structure of butane?

No. A molecular formula gives the numbers and types of atoms but does not specify their connectivity.

What information does CH₃CH₂CH₂CH₃ provide that C₄H₁₀ does not?

It shows that the four carbon atoms are connected consecutively in an unbranched chain.

Carbon Connectivity

The pattern describing which carbon atoms are directly bonded to which other atoms in a molecule.

Atomic Connectivity

The pattern describing which atoms in a molecule are directly bonded to one another.

Does changing the way a molecule is drawn necessarily change its identity?

No. Different drawings can represent the same molecule if the atoms have the same connectivity.

How do you determine whether two differently drawn structures represent the same compound?

Compare their atomic connectivity. If corresponding atoms are connected in the same way, the drawings can represent the same compound.

Can a straight drawing and a bent or zigzag drawing represent the same alkane?

Yes. The shape of the drawing can change while the underlying atomic connectivity remains the same.

Does bending a drawn carbon chain create a new compound?

No. Bending or changing the orientation of a drawing does not create a new compound if connectivity remains unchanged.

Does rotating a structural drawing create a different compound?

No. Rotation changes only how the structure appears on the page, not its atomic connectivity.

What do ordinary two-dimensional structural drawings primarily communicate?

They primarily communicate which atoms are connected to one another rather than necessarily showing the molecule's exact three-dimensional geometry.

Does a zigzag structural drawing necessarily mean the molecule permanently has that exact geometry?

No. Such a drawing primarily represents connectivity and does not by itself specify one fixed three-dimensional geometry.

Can one compound have several valid structural representations?

Yes. The same compound can be represented using expanded, condensed, or differently oriented structural drawings.

Butane as an Expanded Structure

A four-carbon continuous chain in which all carbon and hydrogen atoms and their bonds can be shown explicitly.

Butane as a Condensed Structure

CH₃CH₂CH₂CH₃

Butane Using Parentheses

CH₃(CH₂)₂CH₃

n-Butane

Another notation for normal, straight-chain butane; it distinguishes the straight-chain structure from branched C₄H₁₀.

Meaning of n- in n-Butane

The n- stands for normal and indicates the unbranched, straight-chain form.

What must remain unchanged among different valid representations of the same molecule?

The connectivity of the atoms.

Same Molecular Formula vs Same Compound

Having the same molecular formula does not necessarily mean two structures are the same compound; their atomic connectivity must also be considered.

Same Connectivity vs Same Molecular Formula

Same connectivity describes the same arrangement of bonded atoms, whereas the same molecular formula only guarantees the same numbers and types of atoms.

Why is connectivity important in organic chemistry?

Because compounds with the same molecular formula can be different compounds if their atoms are connected differently.

How can you avoid mistaking two drawings of the same alkane for different compounds?

Trace the carbon skeleton and compare which atoms are bonded to one another instead of judging the structures by their visual orientation.

Carbon Skeleton

The framework formed by the connected carbon atoms of an organic molecule.

Continuous Carbon Chain

A sequence of carbon atoms connected one after another by C–C bonds without interruption.

What should you examine first when comparing two alkane drawings?

Examine the carbon skeleton and determine whether the carbon atoms have the same connectivity.

Representation vs Molecular Identity

A representation is the way a molecule is drawn or written; molecular identity depends on the actual arrangement and connectivity of its atoms.