chap 5 part 3 chap 6 part 1Stereochemistry and Alkyl Halides: Structure, Properties, and Nomenclature

Stereochemistry: Meso Compounds and Chirality

Meso Compounds

Meso compounds are a special class of stereoisomers that contain multiple chiral centers but are overall achiral due to an internal plane of symmetry. As a result, they are optically inactive.

• Definition: A meso compound is a stereoisomer with two or more chiral centers that is superimposable on its mirror image.

• Optical Activity: Meso compounds do not rotate plane-polarized light.

• Symmetry: The presence of a mirror plane (plane of symmetry) within the molecule is key.

• Example: Tartaric acid is a classic example of a meso compound.

Example Structure:

• Consider a molecule with two chiral centers, but with substituents arranged so that the left and right halves are mirror images (e.g., (2R,3S)-2,3-dihydroxybutanedioic acid).

Additional info: Meso compounds are important in stereochemistry because they reduce the number of possible stereoisomers for a given molecular formula.

Chirality and Optical Activity

Chirality is a property of a molecule that makes it non-superimposable on its mirror image. Such molecules are called chiral and can exist as pairs of enantiomers.

• Chiral Center: Typically a carbon atom bonded to four different groups.

• Enantiomers: Non-superimposable mirror images; have identical physical properties except for the direction in which they rotate plane-polarized light and their reactions with other chiral substances.

• Optical Activity: Chiral molecules rotate plane-polarized light; the direction and magnitude are measured as specific rotation.

Assigning (R) and (S) Configuration

The Cahn-Ingold-Prelog priority rules are used to assign absolute configuration to chiral centers as either (R) or (S).

• Step 1: Assign priorities to the four substituents attached to the chiral center based on atomic number (higher atomic number = higher priority).

• Step 2: Orient the molecule so that the lowest priority group is pointing away from you.

• Step 3: Trace a path from priority 1 → 2 → 3. If the path is clockwise, the configuration is (R); if counterclockwise, it is (S).

Example: For a chiral carbon with substituents Br (1), Cl (2), F (3), and H (4), assign priorities and determine the configuration.

Fischer Projections

Fischer projections are a two-dimensional representation of three-dimensional organic molecules, commonly used for sugars and amino acids.

• Horizontal lines: Represent bonds coming out of the plane (towards the viewer).

• Vertical lines: Represent bonds going behind the plane (away from the viewer).

• Advantages: Makes it easier to compare stereochemistry, especially for molecules with multiple chiral centers.

• Limitations: Less intuitive for visualizing 3D structure and for assigning (R)/(S) configuration.

Example: D-glucose and D-galactose differ at a single chiral center, leading to different physical properties (e.g., melting points).

Physical and Chemical Properties of Stereoisomers

• Even a single difference in stereochemistry can lead to significant changes in physical and chemical properties.

• Example Table:

Compound	Melting Point (°C)
D-glucose	148
D-galactose	167

Alkyl Halides: Structure, Nomenclature, and Properties

Definition and Structure

An alkyl halide (haloalkane) is an organic compound containing a halogen atom (F, Cl, Br, I) attached to an sp3-hybridized carbon atom.

• General formula: R–X, where R is an alkyl group and X is a halogen.

• Example: CH3F (methyl fluoride)

Nomenclature of Alkyl Halides

• IUPAC Naming: Name the parent hydrocarbon, identify the halogen as a prefix, and assign the lowest possible number to the halogen substituent.

• Examples:

  • 1-bromobutane

  • 3-iodopentane

  • trans-1,2-dichlorocyclohexane

• Common Names: Some alkyl halides have traditional names, such as isopropyl fluoride, cyclohexyl iodide, methylene chloride (CH2Cl2), and bromoform (CHBr3).

Practice Problems

• Draw the structure for the following names:

  • tert-butyl bromide

  • Z-bromo-3-ethyl-4-methylhexane

  • cis-1-fluoro-3-(fluoromethyl)cyclohexane

  • Chloroform (CHCl3)

Classification of Alkyl Halides

• Primary (1°): Halogen attached to a carbon bonded to one other carbon (RCH2X).

• Secondary (2°): Halogen attached to a carbon bonded to two other carbons (R2CHX).

• Tertiary (3°): Halogen attached to a carbon bonded to three other carbons (R3CX).

• Geminal dihalide: Two halogens on the same carbon.

• Vicinal dihalide: Two halogens on adjacent carbons.

Uses of Alkyl Halides

• Solvents (e.g., methylene chloride)

• Anesthetics (e.g., halothane)

• Refrigerants (e.g., Freons)

• Pesticides (e.g., DDT)

Structure and Bonding in Alkyl Halides

• The carbon-halogen bond is polarized due to the difference in electronegativity between carbon and the halogen.

• As the halogen atom increases in size (from F to I), the bond length increases and the bond becomes less polar.

• Bond Dipole Moments: The dipole moment ($\mu$) is a measure of bond polarity. For C–X bonds:

Bond	Dipole Moment (D)
C–F	1.51
C–Cl	1.56
C–Br	1.48
C–I	1.29

• Bond Lengths: Increase from C–F to C–I as halogen size increases.

• Electronegativity: Decreases from F to I, so C–F is the most polar bond.

Equation for Dipole Moment:

$\mu = 4.8 \times q \times d$

where $q$ is the charge and $d$ is the bond length in angstroms.

Physical Properties of Alkyl Halides

• Boiling Points: Generally increase with molecular weight and polarizability of the halogen.

• Intermolecular Forces: London dispersion forces (LDF) dominate, with some dipole-dipole interactions for more polar alkyl halides.

• Density: Most alkyl halides are denser than hydrocarbons but less dense than water, except for those with multiple halogens (e.g., bromoform, chloroform) which can be denser than water.

Atom	Radius (Å)
F	1.20
Cl	1.35
Br	1.80
I	1.95

Additional info: The C–F bond is similar in length to a C–H bond, making fluorinated compounds important in medicinal chemistry due to their unique properties.