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i-Butane Chemical Formula Guide

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Chemical Formula for i-Butane - Formula Quest Mania

Complete Structure of i-Butane

Isobutane, or i-butane, is one of the most widely studied hydrocarbons in organic chemistry. Its importance extends beyond textbooks and enters practical fields such as fuel engineering, refrigeration technologies, polymer manufacturing, and environmental science. Just like fundamental physics concepts—such as Physics Formula of Earthquake Waves—understanding the chemical formula for i-butane is essential not only for academic purposes but also for industrial processes that rely heavily on hydrocarbon behavior and molecular structure.

This comprehensive article explores the formula of i-butane in depth, covering its molecular formula, structural arrangements, isomerism, chemical reactions, physical properties, environmental impact, industrial significance, and relevant example problems. All explanations are enriched with MathJax-supported equations to ensure clarity and precision.

Introduction to i-Butane

I-butane, also known as isobutane or 2-methylpropane, is a branched-chain alkane. Its molecular formula is identical to n-butane, but its structural arrangement is different. This difference, although small in appearance, leads to significant changes in physical and chemical behavior.

In many organic chemistry courses, i-butane serves as a foundational example for illustrating the concept of structural isomerism. Students learn early on that molecules with the same molecular formula can have completely different shapes, resulting in different properties and reactivities. This distinction is central to understanding the diversity and complexity of organic compounds.

Molecular Formula of i-Butane

The molecular formula of i-butane is:

$$C_4H_{10}$$

This molecular formula corresponds to an alkane with four carbon atoms. However, in chemistry, molecular formulas alone do not provide the full picture. They tell us the number of atoms present but not how they are connected. Therefore, to understand i-butane properly, we must examine its structural formula, condensed formula, and 3D geometry.

General Formula of Alkanes

All alkanes follow the general molecular formula:

$$C_nH_{2n+2}$$

By substituting \( n = 4 \), we obtain:

$$C_4H_{2(4)+2} = C_4H_{10}$$

This confirms that i-butane is a fully saturated hydrocarbon belonging to the alkane family.

Structural Formula of i-Butane

The structural formula provides insight into how atoms are connected. The defining characteristic of i-butane is its branching:

$$CH_3 - CH(CH_3) - CH_3$$

In this structure, one carbon atom connects to three other carbons, forming a central branching point. This is what classifies it as a branched alkane.

Expanded Structural Formula

To visualize all hydrogen atoms explicitly, chemists often draw expanded formulas. These show the tetrahedral shape and full connectivity:

$$ \begin{aligned} & H \\ & | \\ CH_3 - C - CH_3 \\ & | \\ & CH_3 \end{aligned} $$

This representation reveals the carbon skeleton more clearly and highlights the central carbon atom bonding with three methyl groups and one hydrogen atom.

Condensed Structural Formula of i-Butane

In many contexts, especially in organic reaction mechanisms, chemists prefer shorter notations. The condensed structural formula for i-butane is:

$$ (CH_3)_3CH $$

This expresses the molecule's compact, symmetrical nature more efficiently.

Lewis Structure of i-Butane

The Lewis structure illustrates electron pair sharing and bonds between atoms. Though relatively simple, hydrocarbons benefit from this representation for bonding clarity.

A simplified Lewis-style representation can be written as:

$$CH_3 - C(H)(CH_3) - CH_3$$

Each dash represents a pair of shared electrons. Lewis structures reinforce that carbon forms four covalent bonds and hydrogen forms only one.

Three-Dimensional Geometry of i-Butane

All alkanes adopt a tetrahedral geometry around each carbon atom due to the arrangement of electron pairs according to VSEPR theory. I-butane’s central carbon atom is bonded to four substituents (three methyl groups and one hydrogen), creating a perfectly tetrahedral center.

This geometry influences:

  • Boiling point
  • Intermolecular forces
  • Melting point
  • Conformational stability

The compact structure of i-butane results in lower surface area and weaker London dispersion forces than n-butane, which affects many physical properties.

I-Butane vs. n-Butane: A Detailed Comparison

Although both compounds share the same molecular formula, they exhibit markedly different characteristics due to their structural differences.

Structural Difference

n-Butane:

$$CH_3 - CH_2 - CH_2 - CH_3$$

i-Butane:

$$CH_3 - CH(CH_3) - CH_3$$

This structural difference leads to changes in:

  • Boiling point
  • Melting point
  • Combustion properties
  • Molecular symmetry
  • Reactivity in substitution reactions

Boiling Point Comparison

One of the most notable differences is the boiling point:

  • n-Butane: –0.5°C
  • i-Butane: –11.7°C

Because i-butane is more compact and spherical, it has weaker temporary dipoles, resulting in a lower boiling point.

IUPAC Naming of i-Butane

The systematic IUPAC name of i-butane helps classify it formally:

2-methylpropane

This name comes from:

  • Longest chain: propane (3 carbons)
  • Substituent: methyl group
  • Location: carbon number 2

Chemical Properties of i-Butane

Although i-butane is relatively inert under standard conditions, as a saturated hydrocarbon, it undergoes typical alkane reactions such as combustion, halogenation, and cracking.

1. Combustion Reaction

Complete combustion occurs in excess oxygen:

$$C_4H_{10} + \frac{13}{2}O_2 \rightarrow 4CO_2 + 5H_2O$$

This reaction releases heat energy and is used in cooking gas systems and portable lighters.

2. Halogenation Reaction

Halogenation requires UV light and proceeds through a free-radical substitution mechanism:

$$C_4H_{10} + Cl_2 \xrightarrow{hv} C_4H_9Cl + HCl$$

Multiple chlorination steps may produce dichloro-, trichloro-, and tetrachloro- derivatives.

3. Cracking Reaction

Under high temperature:

$$C_4H_{10} \rightarrow C_2H_4 + C_2H_6$$

The products are essential for producing polyethylene and other polymers.

4. Dehydrogenation to Produce Isobutylene

Dehydrogenation forms:

$$C_4H_{10} \rightarrow C_4H_8 + H_2$$

The product, isobutylene, is vital in producing synthetic rubber and fuel additives.

Physical Properties of i-Butane

I-Butane’s physical behavior is closely linked to its branched structure.

  • Molecular mass: 58.12 g/mol
  • Boiling point: –11.7°C
  • Melting point: –159.6°C
  • Density at STP: 2.51 g/L
  • Flash point: –83°C
  • Solubility in water: Very low

Why is i-Butane less stable at high temperatures?

Because its branched structure places more electron density around the central carbon, the molecule is more reactive toward cracking and dehydrogenation at elevated temperatures.

Isomerism in Butane

Butane displays one of the simplest forms of isomerism: chain isomerism.

The two chain isomers are:

Chain isomers differ in carbon skeleton arrangement but share identical molecular formulas.

Examples and Calculations Involving i-Butane

Example 1: Degree of Unsaturation

Formula:

$$DU = \frac{2C + 2 - H}{2}$$

For i-butane:

$$DU = \frac{2(4) + 2 - 10}{2} = 0$$

The molecule is fully saturated.

Example 2: Total σ-Bonds

Carbon–hydrogen bonds: 10 Carbon–carbon bonds: 3

Total:

$$13 \sigma\text{-bonds}$$

Example 3: Heat of Combustion Comparison

Branched alkanes generally release slightly less heat upon combustion per mole because they are more stable.

Thus:

  • n-butane = lower stability, higher heat release
  • i-butane = higher stability, lower heat release

Example 4: Vapor Pressure Calculation Concept

I-butane has a higher vapor pressure at room temperature due to weaker intermolecular forces. This allows it to vaporize more easily, making it suitable as a propellant.

Example 5: Stoichiometric Air Requirement for Combustion

Air contains approximately 21% oxygen. From the combustion reaction:

$$C_4H_{10} + \frac{13}{2}O_2 \rightarrow ...$$

Moles of O2 required per mole of fuel = \(6.5\).

Since air is 21% O2:

Air required:

$$\frac{6.5}{0.21} \approx 30.95 \text{ moles of air}$$

This is critical for fuel-air mixture engineering.

Industrial Applications of i-Butane

I-butane is used extensively in several major industries due to its volatility, stability, and clean-burning characteristics.

1. Refrigeration (R-600a)

I-butane is widely used as a refrigerant because it offers:

  • High energy efficiency
  • Low global warming potential
  • No ozone depletion

2. Fuel Source

It is commonly used in:

  • Lighters
  • Camping stoves
  • Portable fuel canisters

3. Petrochemical Feedstock

I-butane is converted into isobutylene and then used in:

  • Polyisobutylene production
  • Butyl rubber manufacturing
  • High-octane gasoline additives (MTBE)

4. Aerosol Propellant

Its vapor pressure and flammability characteristics make it suitable for:

  • Deodorants
  • Cooking sprays
  • Lubricant sprays

Environmental Impact of i-Butane

While i-butane does not harm the ozone layer, it is still a hydrocarbon and contributes to greenhouse gas formation. However, its environmental footprint is lower than many older refrigerants such as CFCs and HCFCs.

Positive Environmental Aspects

  • Zero ozone depletion
  • Low atmospheric lifetime
  • Better energy efficiency in refrigeration systems

Negative Environmental Aspects

  • Flammability hazard
  • Contributes to smog formation through VOC reactions
  • Mild greenhouse gas effects

Safety Considerations

Since i-butane is flammable, safety practices are essential:

  • Store in airtight, pressure-rated containers
  • Use in ventilated areas
  • Keep away from sparks, flames, and hot surfaces
  • Avoid inhalation

I-butane is a versatile hydrocarbon with the molecular formula \(C_4H_{10}\). Its branched structure gives it unique physical and chemical properties, distinguishing it from n-butane despite having the same molecular formula. Understanding its structure, reactivity, and industrial uses provides valuable insight into the behavior of branched alkanes. As an essential compound in fuel, refrigeration, and petrochemical industries, i-butane remains vital to both scientific study and modern technological applications.

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