Chapter 2 Families of Carbon Compounds. Basic Definitions Hydrocarbons- Compounds containing only carbon and hydrogen. Alkanes- hydrocarbons that contain.

Chapter 2

Families of Carbon Compounds

Basic Definitions

• Hydrocarbons- Compounds containing only carbon and hydrogen.

• Alkanes- hydrocarbons that contain only carbon-carbon single bonds

• Alkenes- hydrocarbons that contain at least one carbon-carbon double bond

• Alkynes- hydrocarbons that contain at least one carbon-carbon triple bond

Basic Definitions, cont

• Saturated hydrocarbons- hydrocarbons that contain no multiple bonds

• Unsaturated hydrocarbons- hydrocarbons that contain multiple bonds such as double or triple bonds

Aromatic Hydrocarbons

• Aromatic Hydrocarbons- unsaturated, cyclic hydrocarbons.

• Benzene is the simplest example• All the bonds of a benzene ring are the same

lengths!!• All the carbons are sp2 hybridized • One lobe of each p orbital extends above the

ring while the other extends below the ring

Polar Covalent Bonds

• Heteroatoms- atoms that form covalent bonds and have unshared pairs of electrons.– Ex.

• When atoms of different electronegativity form covalent bonds, the electrons are not shared equally

• The result is a Polar Covalent Bond

Dipoles

• As a result of the unequal sharing of electrons, a dipole is created.– Ex.

• Dipoles and Dipole moments are important physical properties.

Polar Bonds vs Polar Molecules

• Any diatomic molecule in which the two atoms have different electronegativity will have a dipole moment

• However, just because compounds have polar covalent bonds doesn’t mean they have dipole moments.

• In order for a molecule to be polar, the center of partial positive charge and the center of partial negative charge must be in different locations.

Polar Molecules, cont

• Lone pairs will contribute more than electronegative elements– Ex. Water Ammonia

• Dipole Moments in Alkenes account for the cis/trans isomers having different properties

Functional Groups

• Functional Groups are:– The part of the molecule where most reactions

occur– The part of the molecule that most determines

the chemical and physical properties of a compound

– The basis by which compounds are grouped into families

– The basis for the nomenclature system

Alkyl Groups & the symbol R

• Alkyl group- the groups we identify for the purpose of naming. They are obtained by removing a hydrogen from an Alkane.– Ex.

• The symbol –R is used to generically represent an alkyl group.

The Phenyl Group

• Same concept as an Alkyl group, except the hydrogen is taken off a benzene ring

• There are multiple abbreviations and symbols:

Alkyl Halides

• A compound in which a Halogen replaces one or more of the hydrogens on an Alkane.

• These are also termed haloalkanes.

• They are classified as primary, secondary, and tertiary based of the carbon the halogen is directly bonded to.

Alcohols

• Functional group is –OH attached to a sp3 carbon• They can be viewed as a hydroxyl derivative of an

alkane or an alkyl derivative of water• They are also classified as 1o, 2o, and 3o

Ethers

• General Formula R-O-R or R-O-R’• Derivatives of water where both hydrogens

are replaced with alkyl groups• Usually named by naming alkyl groups and

adding the word ether.• Use di-alkyl if alkyl groups are the same

Amines

• Organic derivatives of ammonia• Also classified as 1o, 2o, and 3o but by different

criteria!• Amines are classified based on the number of

carbons directly bonded to the Nitrogen.• The nitrogen is considered to be sp3 hybridized

with the lone pair in a hybridized orbital.

Aldehydes and Ketones

• Both contain the carbonyl group• Aldehydes have at least one hydrogen bonded

to the carbonyl carbon• Ketones have two carbons bonded to the

carbonyl carbon.• General Formulas:

• Examples:

Carboxylic Acids

• General Formula:

• Functional groups is the carboxyl group

• Examples

Esters

• General Formula

• Widely used in flavors and scents.

Amides

• General Formula:

• Examples

Nitriles

• General Formula:

• Both carbon and nitrogen are sp hybridized

• Usually named by adding the suffix nitrile to the end of the hydrocarbon name

• Examples:

Physical Properties and Structure

• Melting Point and boiling point are easily measured physical properties.

• They are used to identify and isolate organic compounds

• When new substances are made, we have to make reasonably accurate estimates.

• These estimates are based on the structure and the forces that act between molecules and ions

Physical Properties and Structure, cont

• The temperature at which phase changes occur are an indication of the strength of these intermolecular forces.

• Melting Point- the temperature at which equilibrium exist between the well-ordered crystalline state and the more random liquid state

Intermolecular Forces

• Ion-Ion forces- electrostatic attractions between oppositely charged ions.– These forces are so strong that salts typically

decompose before boiling.

• Dipole-Dipole Forces- the attraction between the positive end of one polar molecule and the negative end of another polar molecule.

Intermolecular Forces

• Hydrogen Bonding- very strong dipole-dipole attractions that occur between a hydrogen atom bonded to small, strongly electronegative atoms (O, N, and F) and the lone pair of electrons on another such atom

• Hydrogen Bonds are weaker than a covalent bond but much stronger then a regular dipole-dipole interaction

Intermolecular Forces

• van der Waals/London/Dispersion Forces- the intermolecular attraction that exists for all molecules that arise due to the spontaneous, uneven distribution of electrons in a covalent bond.

Polarizability

• Polarizability- the ability of the electrons to respond to a changing electronic field

• Depends on how tightly or loosely electrons are held

F < Cl < Br < I

Boiling Point

• Boiling Point- the temperature at which the vapor pressure of a liquid equals the pressure of the atmosphere above it.

• Boiling Points are pressure dependant

• Read section 2.13c Boiling Points, about BP’s and factors that affect BP’s

Solubility

• “Like dissolves Like”

• Dissolving something is a lot like melting it

• Ex.

With organic compounds, you have to consider the predominant functionality

Infrared Spectroscopy, IR

• Useful, quick way to identify what functional groups are present

• It is considered to be a fingerprint for organic compounds, in some instances

• Have to be careful, it is a Qualitative technique, not a Quantitative.

Infrared Spectroscopy, IR

• It uses a wide range of frequencies in the infrared region

• Covalent bonds absorb certain frequencies• From this absorption, we know what bonds

are present• Results are graphed as absorption vs

frequency

Infrared Spectroscopy, IR

• The frequency is often in wavenumbers, which is 1/λ (λ= wavelength)

• Absorption is found by subtracting what passed through the sample from what was there when sample was no present

• Covalent bonds are like springs and begin to vibrate when specific energies are absorbed.

Types of Vibrations

• Symmetrical Stretching

• Asymmetric Stretching

• In Plane Bending

• Out of Plane Bending

Infrared Spectroscopy, IR

• There are different vibrational energy levels• When a bond absorbs a specific wavelength of

IR, it becomes excited and moves from one energy level to another.

• The frequency of a stretching vibration can be related to two factors:– The masses of the bonded atoms– The relative stiffness of the bond

Infrared Spectroscopy, IR

• Not all bonds are seen in IR

• To be seen by IR, the dipole moment of a bond must change during vibration

• General rule, symmetrical bonds do not show stretching peaks.

Absorptions

• Table 2.6, page 89

Two types of IR problems

1) Predict spectrum from structure

2) Using Molecular Formula and Spectra, predict the structure

Spectrum from Structure

1) List all bond types

2) List frequency ranges

3) Draw spectrum

4) Label peaks

Using Molecular Formula and spectra to get structure

1) Calculate Double Bond Equivalents (DBE’s)

2) Draw possible structures

3) Identify major peaks in spectrum and match to possible structures

Helpful suggestions:

a) Check region around 3000b) Is there a strong, broad peak around 3500c) Is there a sharp peak in range of 1630-1780

• Review Applications of Basic Principles, page 97