MC

Alkane Naming and Stereochemistry

Alkane Naming Conventions

Choosing the Parent Chain

  • Longest Chain Rule: The first step in naming an alkane is to identify and choose the longest continuous carbon chain, which serves as the parent chain. For example, a six-carbon parent chain implies a "hexane."
  • Maximum Number of Substituents Rule: When a molecule offers different continuous carbon chains of the same length, the parent chain selected must be the one that has the maximum number of substituents attached to it. This is crucial for correct nomenclature.
    • Example: If one C6 chain has one isopropyl substituent, and another C6 chain has one ethyl and one methyl substituent, the latter (with two substituents) is the correct parent chain choice.

Numbering the Parent Chain

  • Lower Number Rule: Number the parent chain carbons in a way that gives the substituents the lowest possible numbers.
  • Alphabetical Priority for Numbering: If two different numbering schemes result in the same set of substituent numbers, then assign the lower number to the substituent that comes first alphabetically.
    • Example: For a cyclohexane with a butyl group and a methyl group, if both numbering directions result in "1 and 3" for the substituents, the butyl group (B) should be assigned C1 and the methyl group (M) assigned C3 because 'B' comes before 'M' alphabetically.

Naming Substituents

  • Alkyl Groups: Groups like methyl (CH3), ethyl (C2H5), propyl (C3H7), isopropyl (CH(CH3)_2).
    • Special Butyl Groups: Butyl (C4H9).
      • Sec-butyl: When the parent chain attaches to the second carbon of a four-carbon chain (CH(CH3)CH2CH_3).
      • Tert-butyl: When the parent chain attaches to the central carbon of a three-methyl group (C(CH3)3).
      • Isobutyl: When the parent chain attaches to the first carbon of a branched three-carbon chain (CH2CH(CH3)_2).
  • Halogens: Bromo (Br), Fluoro (F), Chloro (Cl), Iodo (I).
  • Other Common Substituents (Memorization Recommended for Exams):
    • Hydroxy (OH)
    • Amino (NH_2)
    • Nitro (NO_2)
    • Cyano (CN)
    • Vinyl (CH=CH_2)
    • Allyl (CH2CH=CH2)
    • Phenyl (C6H5 ring)
    • Benzyl (CH2C6H5 ring attached to a CH2 group)

Alphabetical Ordering of Substituents

  • Rule: When listing substituents in the final name, arrange them alphabetically.
  • Ignoring Prefixes: Do not consider prefixes like "di," "tri," "tetra," "sec," or "tert" when determining alphabetical order.
    • Example: "Ethyl" comes before "dimethyl" because 'e' comes before 'm' (ignore 'di').
    • Example: "Sec-butyl" comes before "methyl" because 'b' comes before 'm' (ignore 'sec').

Naming Cyclic Alkanes

  • Ring as Parent Chain: If the ring contains more carbons than any linear chain attached to it, or if there's only one substituent, the cyclic alkane becomes the parent chain (e.g., "cyclohexane," "cyclopentane").
  • Substituent Naming: Attached groups are named as substituents.
    • Example: A propyl group attached to cyclohexane is named "propylcyclohexane" (or "1-propylcyclohexane"; the '1' is often omitted for a single substituent).
  • Multiple Substituents: Number the ring to give substituents the lowest possible numbers and follow alphabetical rules for ordering and numbering priority.
    • Example: For a cyclopentane with ethyl, dimethyl, and propyl groups: Four ethyl, two and four dimethyl, three propyl cyclopentane (incorrect example from transcript, actual order would be 1-ethyl-2,4-dimethyl-3-propylcyclopentane). Alphabetical order (E, then M, then P) is followed.

Naming Bicyclic Compounds (Informational Only)

  • Structure: Composed of two rings sharing two or more atoms. "Bicyclo" prefix is used.
  • Numbering: Follows a specific system based on the number of carbons in each bridge connecting the bridgehead carbons.
    • Example: "Bicyclo[4.3.0]nonane" indicates two bridgehead carbons with bridges of 4, 3, and 0 carbons connecting them, forming a total of 9 carbons.
  • Note: This rule is presented for information and not explicitly stated to be for exam/quiz memorization.

Stereoisomerism and Molecular Conformations

Types of Molecular Movement

  • Stretching/Vibration: Changes in bond length, requiring lower energy.
  • Bending: Changes in bond angle, requiring lower energy.
  • Rotation: Movement around single bonds, crucial in chemistry for understanding molecular shapes and reactivity.

Stereochemistry: Isomers and Conformations

  • Definition of Stereoisomers: Molecules that have the same molecular formula and the same connectivity (atoms are bonded in the same order) but differ in the three-dimensional arrangement of their atoms in space. They are not constitutional (structural) isomers.
    • Example: Cis and trans isomers of cyclic alkanes are stereoisomers.
  • Ethane Example: Ethane (CH3-CH3) has no constitutional isomers as there's only one way to connect its two carbons. However, it exhibits stereoisomerism due to rotation around the carbon-carbon single bond.
  • Conformations: Different spatial arrangements of a molecule that result from rotation around single bonds are called conformations.

Dihedral (Torsion) Angle

  • Bond Angle vs. Torsion Angle: A bond angle refers to the angle between two bonds originating from the same carbon atom. A dihedral angle (or torsion angle) refers to the angle between two planes, specifically the angle between a bond on a front carbon and a bond on an adjacent back carbon.
  • Ethane Conformations: Ethane has two primary conformations:
    • Eclipsed Conformation: Torsion angle is 0^
      between groups on adjacent carbons. Hydrogens on the front carbon directly overlap with those on the back carbon when viewed down the C-C bond.
    • Staggered Conformation: Torsion angle is 60^
      between groups on adjacent carbons. Hydrogens on the front carbon are positioned halfway between the hydrogens on the back carbon when viewed down the C-C bond.

Cis and Trans Isomers in Cyclic Compounds

  • Definition: A type of stereoisomerism observed in cyclic molecules (and sometimes alkenes) where substituents are on the same side or opposite sides of the ring plane.
  • Cis Isomer: Substituents are on the same side of the ring. This is visually represented by both substituents using wedges (both 'top') or both using dashes (both 'bottom').
    • Example: Cis-1,3-dimethylcyclohexane (both methyl groups on the same side).
  • Trans Isomer: Substituents are on opposite sides of the ring. Visually represented by one substituent using a wedge and the other using a dash (one 'top', one 'bottom').
    • Example: Trans-1,3-dimethylcyclohexane (one methyl group on top, the other on bottom).
  • Identifying Cis/Trans: When analyzing structures, identify if substituent bonds are both wedges/both dashes (cis) or one wedge/one dash (trans).