Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure

Problem 32

Chapter 4, Problem 32

What is the smallest straight-chain alkane that is a liquid at room temperature (which is about 25 °C)?

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Understand the problem: We are tasked with identifying the smallest straight-chain alkane that exists as a liquid at room temperature (approximately 25 °C). Alkanes are hydrocarbons with the general formula C_nH_2n+2, and their physical state depends on molecular size and intermolecular forces.

Recall the trend: The physical state of alkanes (gas, liquid, or solid) at room temperature depends on their molecular weight and boiling point. Smaller alkanes (with fewer carbon atoms) tend to be gases, while larger alkanes are liquids or solids due to stronger London dispersion forces.

Identify the boiling point range: For an alkane to be a liquid at room temperature, its boiling point must be between 25 °C and 100 °C. Alkanes with boiling points below 25 °C are gases, and those above 100 °C are typically liquids or solids.

Consult boiling point data: Look up the boiling points of straight-chain alkanes (e.g., methane, ethane, propane, butane, pentane, hexane, etc.) to determine the smallest alkane whose boiling point falls within the liquid range at room temperature.

Conclude the smallest alkane: Based on the boiling point data, identify the straight-chain alkane with the lowest molecular weight that meets the criteria of being a liquid at room temperature.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Alkanes

Alkanes are saturated hydrocarbons consisting only of carbon and hydrogen atoms, connected by single bonds. They follow the general formula CnH2n+2, where 'n' is the number of carbon atoms. Alkanes can be straight-chain or branched, and their physical properties, such as boiling and melting points, vary with molecular size.

Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure, causing it to change from liquid to gas. For alkanes, the boiling point increases with molecular weight due to greater van der Waals forces. Understanding boiling points helps determine the state of an alkane at a given temperature.

State of Matter at Room Temperature

Room temperature, typically around 25 °C, is a standard reference point for assessing the physical state of substances. Alkanes with fewer than five carbon atoms (like methane and ethane) are gases at this temperature, while those with five or more carbons are liquids. Identifying the smallest alkane that is liquid at room temperature requires knowledge of these state changes.

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