Organic Chemistry (CHM 221) Chapter 4
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Questions and Answers

What is the general formula for acyclic alkanes?

CnH2n+2

What type of bond do all alkanes have?

  • Sigma bonds (correct)
  • Double bonds
  • Pi bonds
  • Triple bonds

    • Cycloalkanes are named by adding the prefix "cyclo-" to the name of the acyclic alkane with the same number of carbons.

    True

    What is the name of the IUPAC system used for naming organic molecules?

    <p>International Union of Pure and Applied Chemistry</p> Signup and view all the answers

    What three parts make up the name of an organic molecule?

    <p>parent name, suffix, prefix</p> Signup and view all the answers

    Match the following terms with their descriptions.

    <p>Alkyl group = A carbon substituent bonded to a longer carbon chain. Constitutional isomer = A compound with the same molecular formula but a different arrangement of atoms. Torsional strain = An increase in energy caused by eclipsing interactions. Steric strain = An increase in energy when non-bonded atoms are forced too close together. Dihedral angle = The angle between two planes, particularly used to describe the relationship between atoms in a molecule.</p> Signup and view all the answers

    Alkanes are soluble in water.

    <p>False</p> Signup and view all the answers

    The boiling point of alkanes increases as the number of carbons in the molecule increases.

    <p>True</p> Signup and view all the answers

    What is the primary factor that determines the stability of different conformations of acyclic alkanes?

    <p>Both torsional strain and steric strain</p> Signup and view all the answers

    The staggered conformation of ethane has a 0° dihedral angle.

    <p>False</p> Signup and view all the answers

    The staggered conformation is higher in energy than the eclipsed conformation.

    <p>False</p> Signup and view all the answers

    What is the name of the end-on representation for conformations, first described by Melvin Newman?

    <p>Newman projection</p> Signup and view all the answers

    What are the two types of staggered conformation referred to?

    <p>anti and gauche</p> Signup and view all the answers

    What is angle strain?

    <p>Bond angles deviate from the tetrahedral angle of 109.5°</p> Signup and view all the answers

    Cyclopropanes are known for having a high degree of angle strain.

    <p>True</p> Signup and view all the answers

    Cyclohexanes adopt a puckered "chair" conformation, which is more stable than any other conformation.

    <p>True</p> Signup and view all the answers

    What are the two types of hydrogens present on each carbon in cyclohexane?

    <p>axial and equatorial</p> Signup and view all the answers

    Ring-flipping is a conformational change that interconverts axial and equatorial H atoms in cyclohexanes.

    <p>True</p> Signup and view all the answers

    Larger substituents are more stable in the equatorial position.

    <p>True</p> Signup and view all the answers

    Both conformations of cis-1,4-dimethylcyclohexane are equally stable.

    <p>True</p> Signup and view all the answers

    The diaxial conformation of trans-1,4-dimethylcyclohexane is more stable than the di-equatorial conformation.

    <p>False</p> Signup and view all the answers

    Alkanes can undergo two basic reactions: oxidation and combustion.

    <p>True</p> Signup and view all the answers

    Combustion of alkanes is an example of a reduction reaction.

    <p>False</p> Signup and view all the answers

    What are two things that are produced during the combustion of alkanes?

    <p>carbon dioxide and water</p> Signup and view all the answers

    Study Notes

    Organic Chemistry - Chapter 4: Alkanes

    • Alkanes are aliphatic hydrocarbons containing only carbon and hydrogen, with single (sigma) bonds.
    • Acyclic alkanes have straight chains and follow the general formula CnH2n+2.
    • Cycloalkanes form rings and follow the general formula CnH2n.
    • Nomenclature is not emphasized in this course, but it is important for specifying location, type of molecule, and functional groups.
    • Alkanes are nonpolar due to C-C and C-H bonds.
    • This results in weak van der Waals forces and affects solubility, boiling point, and melting point.
    • Alkanes are soluble in organic solvents and insoluble in water.
    • Boiling and melting points increase with the number of carbons due to increased surface area.
    • Branching decreases boiling and melting points.
    • Increased symmetry increases melting points.
    • Conformations are different arrangements of atoms that are interconverted by rotation about single bonds.
    • Eclipsed conformations have C-H bonds directly aligned.
    • Staggered conformations have C-H bonds bisecting the H-C-H bond angle.
    • Newman Projections are end-on representations of conformations along a C-C bond.
    • Torsional strain increases energy during eclipsing interactions.
    • The energy difference between staggered and eclipsed conformations is called torsional energy.
    • Staggered conformations are more stable than eclipsed conformations.
    • The staggered conformations are more stable (lower in energy) than the eclipsed conformations.
    • 1,2-dimethylcyclopentanes can be cis or trans.
    • Cis isomers have two identical groups on the same side of the ring.
    • Trans isomers have two identical groups on opposite sides of the ring.

    Substituent Naming

    • Alkyl groups, derived from alkanes, are named by changing the -ane suffix to -yl.
      • Methane becomes methyl (CH3—)
      • Ethane becomes ethyl (CH3CH2—)
    • Parent hydrocarbons have different hydrogen atoms. Removal of these different hydrogen atoms forms different alkyl groups, with different names such as propyl or isopropyl by different carbon positions.
    • Naming four-carbon alkyl groups is more complex; giving butyl, sec-butyl, isobutyl and tert-butyl.

    Cycloalkanes

    • Cycloalkanes contain carbons joined in rings.
    • Their general formula is CnH2n, having two fewer hydrogens than their acyclic analogs.
    • Cyclopropanes, cyclobutanes etc., deviate from the tetrahedral angle (109.5°), requiring the molecules to be puckered to relieve strain.
    • Cyclohexane usually adopts a chair conformation, minimizing angle and torsional strain.
    • Cyclohexane has axial and equatorial hydrogens.

    Nomenclature

    • The name of every organic molecule has 3 parts: parent name, a suffix, and prefixes.
    • The parent name indicates the number of carbons in the longest continuous carbon chain.
    • The suffix indicates the functional group.
    • The prefix tells us the identity, position, and number of substituents attached to the carbon chain.
      • Parent chains (lengths in terms of carbons), the appropriate suffix and substituents are used to name complicated molecules.

    Reactions of Alkanes

    • Alkanes undergo oxidation and reduction reactions.
    • Oxidation results in an increase in the number of C-Z bonds and a decrease in the number of C-H bonds.
    • Reduction results in an increase in the number of C-H bonds and a decrease in the number of C-Z bonds.
    • Alkanes undergo combustion (reaction with oxygen) to form carbon dioxide and water.

    Summary

    • The number of carbons affects physical properties (boiling/melting points, solubility).
    • Nomenclature is important for specifying structure, including location and substituent characteristics.
    • Conformational differences (eclipsed/staggered, chair/boat) affect stability.
    • Substituents affect the stability of chair conformations.
    • Alkanes typically undergo combustion or oxidation reactions.

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