Organic Chemistry Chapter 4: Alkanes

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Questions and Answers

What distinguishes branched alkanes from straight-chain alkanes?

Branched alkanes can only be formed from propane.

Branched alkanes have at least one carbon attached to more than two other carbons. (correct)

Branched alkanes have a higher molecular weight than straight-chain alkanes.

Branched alkanes are completely circular in shape.

What change occurs when naming unbranched alkyl groups from alkanes?

The term 'alkyl' is added before the name.

The suffix -ane is replaced with -yl. (correct)

The prefix is removed completely.

The suffix -ane is replaced with -al.

How did the IUPAC contribute to the naming of chemical compounds by the end of the 19th century?

They introduced a trademark system for naming chemical compounds.

They created a systematic nomenclature to ensure uniqueness for each compound. (correct)

They focused solely on the physical properties of compounds.

They eliminated all common names for simplicity.

What is a characteristic property of constitutional isomers?

They possess different melting points and boiling points. Signup and view all the answers

Why does the number of constitutional isomers increase significantly with more carbon atoms?

Increased carbon count allows for more arrangements of connectivity. Signup and view all the answers

Which of the following correctly identifies the parent chain in a branched-chain alkane?

The longest continuous chain of carbons. Signup and view all the answers

What is true about straight-chain alkanes?

They have a zig-zag orientation in their straight form. Signup and view all the answers

How has nomenclature for chemical compounds evolved over time?

It has transitioned from nonsystematic to systematic nomenclature. Signup and view all the answers

What is the correct procedure for numbering the longest carbon chain in a branched alkane?

Start numbering from the end of the chain that is closest to the substituent. Signup and view all the answers

Which prefix is used to indicate the presence of two identical substituents?

Di- Signup and view all the answers

When selecting the parent chain between two chains of equal length, what determines the choice?

The chain with more substituents. Signup and view all the answers

What is the name given to alcohols with two hydroxyl groups?

Diols in IUPAC nomenclature. Signup and view all the answers

In the nomenclature of cycloalkanes, when two substituents are present, which is given position 1?

The substituent with alphabetical priority. Signup and view all the answers

What characterizes the chair conformation of cyclohexane?

It has no ring strain and all C-H bonds are staggered. Signup and view all the answers

What type of strain does cyclopropane primarily experience?

Ring strain due to high angle and tortional strain. Signup and view all the answers

What feature does a bicycloalkane name include?

The specific number of carbons in each bridge. Signup and view all the answers

How are alkenes named according to IUPAC rules?

By identifying the longest chain containing the double bond and changing the suffix from -ane to -ene. Signup and view all the answers

Which conformation of butane is less stable and why?

Gauche, due to steric strain. Signup and view all the answers

In cis-trans isomerism, when are two identical groups referred to as cis?

When they are on the same side of the double bond. Signup and view all the answers

In terms of stability, what position does a methyl group prefer in methylcyclohexane?

Equatorial position, to avoid 1,3-diaxial interactions. Signup and view all the answers

What is a common way to produce alkanes from alkenes?

Hydrogenation. Signup and view all the answers

What type of analysis works backward from a target molecule to a simple starting material?

Retrosynthetic Analysis. Signup and view all the answers

Study Notes

Shapes of Alkanes

"Straight-chain" alkanes adopt a zig-zag orientation in their most linear form, also referred to as unbranched alkanes.
Branched alkanes contain at least one carbon atom connected to more than two other carbons.
Constitutional isomers have the same molecular formula but differ in the connectivity of their atoms, leading to unique physical properties (melting point, boiling point, density).
The number of constitutional isomers increases rapidly as the number of carbons rises.

IUPAC Nomenclature

Prior to the end of the 19th century, compounds were named with nonsystematic, common names based often on their source or properties.
IUPAC began creating a systematic nomenclature in 1892 to ensure unique and unambiguous names for different compounds.
Unbranched alkyl groups are derived from alkanes by removing a hydrogen atom and changing the suffix from -ane to -yl.

Naming Branched-Chain Alkanes

Identify the longest continuous carbon chain as the parent chain, which determines the parent name.
Number the chain starting from the end closest to any substituent to designate their locations.
List substituents alphabetically and use prefixes (di-, tri-, tetra-) for identical substituents.
In cases of competitive chains, choose the one with the greatest number of substituents or the lower number at the first point of difference.

Classification of Hydrogen Atoms

The classification of hydrogen atoms is based on the carbons to which they are attached.

Nomenclature of Alkyl Halides

In IUPAC nomenclature, halides are treated as substituents on the parent chain, sharing equal ranking with alkyl substituents.
Common nomenclature for simple alkyl halides remains accepted by IUPAC.

Nomenclature of Alcohols

The longest carbon chain containing a hydroxyl group is selected, changing the suffix from -ane to -ol.
The hydroxyl must receive the lowest possible number during numbering, with other substituents following accordingly.
Alcohols with two hydroxyl groups are termed diols, referred to as glycols in common nomenclature.

Nomenclature of Cycloalkanes

The prefix "cyclo-" is added to the name of the alkane corresponding to the number of carbons in the ring.
Presence of one substituent implies it is at position one without numbering; for two substituents, the one with the alphabetical priority is numbered first.
Hydroxyl groups hold higher priority than alkyl groups in numbering.

Bicyclic Compounds

Bicycloalkanes consist of 2 fused or bridged rings and use "bicyclo-" as a prefix, with the number of carbons in each bridge noted in square brackets.

Nomenclature of Alkenes

Alkenes are identified by the longest chain containing a double bond, changing the suffix from -ane to -ene and giving the lowest number to the double bond.
Cycloalkenes must have double bonds in positions 1 and 2.

Physical Properties of Alkanes

Boiling points of unbranched alkanes increase with the number of carbons; melting points follow an alternating pattern depending on carbon chain parity.

Sigma Bonds and Bond Rotation

Ethane allows free rotation around the carbon-carbon bond, characterized by staggered and eclipsed conformations.
The staggered conformation is energetically more stable than the eclipsed form, with a difference of 12 kJ mol⁻¹.

Conformational Analysis of Butane

Butane’s rotation gives six key conformations. The gauche conformation is less stable than the anti conformation by 3.8 kJ mol⁻¹ due to repulsive forces.

Ring Strain in Cycloalkanes

Cyclohexane exhibits no ring strain, whereas cyclopropane and cyclobutane both experience angle strain and torsional strain.
Cyclopropane has high strain levels due to its geometric structure.

Conformations of Cyclohexane

Chair conformation is the most stable, free from ring strain, while the boat and twist conformations have increasing instability due to interactions and strain.

Stability of Substituted Cyclohexanes

Axial and equatorial hydrogens impact stability, with equatorial positions being more favorable.
Trans-1,4-dimethylcyclohexane favors a diequatorial conformation, whereas cis-1,4-dimethylcyclohexane prefers an axial-equatorial structure.

Synthesis of Alkanes and Cycloalkanes

Methods include hydrogenation of alkenes and alkynes, reduction of alkyl halides, and alkylation of terminal alkynes.

Retrosynthetic Analysis

The planning of organic synthesis is executed backward from the target molecule to simpler starting materials, often with multiple potential schemes available.

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Description

Explore the fascinating world of alkanes in Chapter 4, focusing on their nomenclature, conformational analysis, and basic synthesis. Learn about the structural shapes of straight-chain and branched alkanes, and how their arrangements affect their properties and reactions.