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Carbon-carbon Covalent Bonds

Carbon forms strong covalent bonds to other carbons and to other elements such as hydrogen, oxygen, nitrogen and sulfur

This accounts for the vast variety of organic compounds possible

Organic compounds are grouped into functional group families

A functional group is a specific grouping of atoms (e.g. carbon- carbon double bonds are in the family of alkenes)
An instrumental technique called infrared (IR) spectroscopy is used to determine the presence of specific functional groups

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Hydrocarbons: Representative Alkanes, Alkenes Alkynes, and Aromatic Compounds

Hydrocarbons contain only carbon and hydrogen atoms
Subgroups of Hydrocarbons:

Alkanes contain only carbon-carbon single bonds
Alkenes contain one or more carbon-carbon double bonds
Alkynes contain one or more carbon-carbon triple bonds
Aromatic hydrocarbons contain benzene-like stable structures (discussed later)

Saturated hydrocarbons: contain only carbon-carbon single bonds e.g. alkanes
Unsaturated hydrocarbons: contain double or triple carbon-carbon bonds e.g. alkene, alkynes, aromatics

Contain fewer than maximum number of hydrogens per carbon
Capable of reacting with H₂ to become saturated

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Representative Hydrocarbons

Alkanes

Principle sources of alkanes are natural gas and petroleum

Smaller alkanes (C₁ to C₄) are gases at room temperature

Methane is

A component of the atmosphere of many planets
Major component of natural gas
Produced by primitive organisms called methanogens found in mud, sewage and cows’ stomachs

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Alkenes

Ethene (ethylene) is a major industrial feedstock

Used in the production of ethanol, ethylene oxide and the polymer polyethylene

Propene (propylene) is also very important in industry

Molecular formula C₃H₆
Used to make the polymer polypropylene and is the starting material for acetone

Many alkenes occur naturally

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Alkynes

Ethyne (acetylene) is used in welding torches because it burns at high temperature

Many alkynes are of biological interest

Capillin is an antifungal agent found naturally
Dactylyne is a marine natural product
Ethinyl estradiol is a synthetic estrogen used in oral contraceptives

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Benzene: A Representative Hydrocarbon

Benzene is the prototypical aromatic compound

The Kekulé structure (named after August Kekulé who formulated it) is a six-membered ring with alternating double and single bonds

Benzene does not actually have discreet single and double carbon-carbon bonds

All carbon-carbon bonds are exactly equal in length (1.38 Å)
This is between the length of a carbon-carbon single bond and a carbon-carbon double bond

Resonance theory explains this by suggesting there are two resonance hybrids that contribute equally to the real structure

The real structure is often depicted as a hexagon with a circle in the middle

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Molecular orbital theory explains the equal bond lengths of benzene by suggesting there in a continuous overlap of p orbitals over the entire ring

All carbons in benzene are sp² hybridized

Each carbon also has a p orbital

Each p orbital does not just overlap with one adjacent p but overlaps with p orbitals on either side to give a continuous bonding molecular orbital that encompasses all 6 carbons
All 6 π electrons are therefore delocalized over the entire ring and this results in the equivalence of all of the carbon-carbon bonds

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Polar Covalent Bonds

Polar covalent bonds occur when a covalent bond is formed between two atoms of differing electronegativities

The more electronegative atom draws electron density closer to itself
The more electronegative atom develops a partial negative charge (δ-) and the less electronegative atom develops a partial positive charge (δ+)
A bond which is polarized is a dipole and has a dipole moment
The direction of the dipole can be indicated by a dipole arrow

The arrow head is the negative end of a dipole, the crossed end is the positive end

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Example: the molecule HCl

The more electronegative chlorine draws electron density away from the hydrogen

Chlorine develops a partial negative charge

The dipole moment of a molecule can be measured experimentally

It is the product of the magnitude of the charges (in electrostatic units: esu) and the distance between the charges (in cm)
The actual unit of measurement is a Debye (D) which is equivalent to 1 x 10^-18 esu cm

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A map of electrostatic potential (MEP) is a way to visualize distribution of charge in a molecule

Parts of the molecule which are red have relatively more electron density or are negative

These region would tend to attract positively charged species

Parts of the molecule which are blue have relatively less electron density or are positive

These region would tend to attract negatively charged species

The MEP is plotted at the van Der Waals surface of a molecule

This is the farthest extent of a molecule’s electron cloud and therefore indicates the shape of the molecule

The MEP of hydrogen chlorine clearly indicates that the negative charge is concentrated near chlorine

The overall shape of the molecule is also represented

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Molecular Dipole

In diatomic molecules a dipole exists if the two atoms are of different electronegativity
In more complicated molecules the molecular dipole is the sum of the bond dipoles
Some molecules with very polar bonds will have no net molecular dipole because the bond dipoles cancel out

The center of positive charge and negative charge coincide in these molecules

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Examples

In carbon tetrachloride the bond dipoles cancel and the overall molecular dipole is 0 Debye

In chloromethane the C-H bonds have only small dipoles but the C-Cl bond has a large dipole and the molecule is quite polar

An unshared pair of electrons on atoms such as oxygen and nitrogen contribute a great deal to a dipole

Water and ammonia have very large net dipoles

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Some cis-trans isomers differ markedly in their dipole moment

In trans 1,2-dichloroethene the two carbon-chlorine dipoles cancel out and the molecular dipole is 0 Debye
In the cis isomer the carbon-chlorine dipoles reinforce and there is a large molecular dipole

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Functional Groups

Functional group families are characterized by the presence of a certain arrangement of atoms called a functional group
A functional group is the site of most chemical reactivity of a molecule

The functional group is responsible for many of the physical properties of a molecule

Alkanes do not have a functional groups

Carbon-carbon single bonds and carbon-hydrogen bonds are generally very unreactive

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Alkyl Groups and the Symbol R

Alkyl groups are obtained by removing a hydrogen from an alkane
Often more than one alkyl group can be obtained from an alkane by removal of different kinds of hydrogens

R is the symbol to represent a generic alkyl groups

The general formula for an alkane can be abbreviated R-H

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A benzene ring with a hydrogen removed is called a phenyl and can be represented in various ways

Toluene (methylbenzene) with its methyl hydrogen removed is called a benzyl group

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Alkyl Halides

In alkyl halides, halogen (F, Cl, Br, I) replaces the hydrogen of an alkane
They are classified based on the carbon the halogen is attached to

If the carbon is attached to one other carbon that carbon is primary (1^o) and the alkyl halide is also 1^o
If the carbon is attached to two other carbons, that carbon is secondary (2^o) and the alkyl halide is 2^o
If the carbon is attached to three other carbons, the carbon is tertiary (3^o) and the alkyl halide is 3^o

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Alcohols

In alcohols the hydrogen of the alkane is replaced by the hydroxyl (-OH) group

An alcohol can be viewed as either a hydroxyl derivative of an alkane or an alkyl derivative of water

Alcohols are also classified according to the carbon the hydroxyl is directly attached to

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Ethers

Ethers have the general formula R-O-R or R-O-R’ where R’ is different from R

These can be considered organic derivatives of water in which both hydrogens are replaced by organic groups
The bond angle at oxygen is close to the tetrahedral angle

Amines

Amines are organic derivatives of ammonia

They are classified according to how many alkyl groups replace the hydrogens of ammonia
This is a different classification scheme than that used in alcohols

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Aldehydes and Ketones

Both contain the carbonyl group

Aldehydes have at least one carbon attached to the carbonyl group

Ketones have two organic groups attached to the carbonyl group

The carbonyl carbon is sp² hybridized

It is trigonal planar and has bond angle about 120^o

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Carboxylic Acids, Esters and Amides

All these groups contain a carbonyl group bonded to an oxygen or nitrogen
Carboxylic Acids

Contain the carboxyl (carbonyl + hydroxyl) group

Esters

A carbonyl group is bonded to an alkoxyl (OR’) group

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Amide

A carbonyl group is bonded to a nitrogen derived from ammonia or an amine

Nitriles

An alkyl group is attached to a carbon triply bonded to a nitrogen

This functional group is called a cyano group

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Summary of Important Families of Organic Compounds

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Summary (cont.)

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Physical Properties and Molecular Structure

The strength of intermolecular forces (forces between molecules) determines the physical properties (i.e. melting point, boiling point and solubility) of a compound
Stronger intermolecular forces result in high melting points and boiling points

More energy must be expended to overcome very strong forces between molecules

The type of intermolecular forces important for a molecule are determined by its structure
The physical properties of some representative compounds are shown on the next slide

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Ion-Ion Forces

Ion-ion forces are between positively and negatively charged ions
These are very strong forces that hold a solid compound consisting of ions together in a crystalline lattice

Melting points are high because a great deal of energy is required to break apart the crystalline lattice

Boiling points are so high that organic ions often decompose before they boil
Example: Sodium acetate

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Dipole-Dipole Forces

Dipole-dipole forces are between molecules with permanent dipoles

There is an interaction between δ+ and δ- areas in each molecule; these are much weaker than ion-ion forces
Molecules align to maximize attraction of δ+ and δ- parts of molecules
Example: acetone

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Hydrogen Bonds

Hydrogen bonds result from very strong dipole-dipole forces
There is an interaction between hydrogens bonded to strongly electronegative atoms (O, N or F) and nonbonding electron pairs on other strongly electronegative atoms (O, N or F)

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Example

Ethanol (CH₃CH₂OH) has a boiling point of +78.5^oC; its isomer methyl ether (CH₃OCH₃) has a boiling point of -24.9^oC

Ethanol molecules are held together by hydrogen bonds whereas methyl ether molecules are held together only by weaker dipole-dipole interactions

A factor in melting points is that symmetrical molecules tend to pack better in the crystalline lattice and have higher melting points

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van der Waals Forces (London or Dispersion Forces)

Van der Waals forces result when a temporary dipole in a molecule caused by a momentary shifting of electrons induces an opposite and also temporary dipole in an adjacent molecule

These temporary opposite dipoles cause a weak attraction between the two molecules
Molecules which rely only on van der Waals forces generally have low melting points and boiling points

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Polarizability predicts the magnitude of van der Waals Interactions

Polarizability is the ability of the electrons on an atom to respond to a changing electric field
Atoms with very loosely held electrons are more polarizable
Iodine atoms are more polarizable than fluorine atoms because the outer shell electrons are more loosely held
Atoms with unshared electrons are more polarizable (a halogen is more polarizable than an alkyl of similar size)

All things being equal larger and heavier molecules have higher boiling points

Larger molecules need more energy to escape the surface of the liquid
Larger organic molecules tend to have more surface area in contact with each other and so have stronger van der Waals interactions
Methane (CH₄) has a boiling point of -162^oC whereas ethane (C₂H₆) has a boiling point of -88.2^oC

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Solubilities

Water dissolves ionic solids by forming strong dipole-ion interactions

These dipole-ion interactions are powerful enough to overcome lattice energy and interionic interactions in the solid

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Generally like dissolves like

Polar solvents tend to dissolve polar solids or polar liquids
Methanol (a water-like molecule) dissolves in water in all proportions and interacts using hydrogen-bonding to the water

A large alkyl group can overwhelm the ability of the polar group to solubilize a molecule in water

Decyl alcohol is only slightly soluble in water
The large alkyl portion is hydrophobic (“water hating”) and overwhelms the capacity of the hydrophilic (“water loving”) hydroxyl

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Generally one hydrophilic group (e.g. hydroxyl) can make a compound with 3 carbons completely soluble in water

One hydrophilic group can make a 5 carbon compound at least partially soluble
A compound is water soluble if at least 3g of it will dissolve in 100 mL water

Summary of Attractive Electric Forces

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Infrared Spectroscopy: An Instrumental Method for Detecting Functional Groups

Electromagnetic radiation in the infrared (IR) frequency range is absorbed by a molecule at certain characteristic frequencies

Energy is absorbed by the bonds in the molecule and they vibrate faster
The bonds behave like tiny springs connecting the atoms

The bonds can absorb energy and vibrate faster only when the added energy is of a particular resonant frequency

The frequencies of absorption are very characteristic of the type of bonds contained in the sample molecule
The type of bonds present are directly related to the functional groups present
A plot of these absorbed frequencies is called an IR spectrum

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Infrared Spectrometer

An infrared spectrometer detects the frequencies absorbed by the sample molecule
Light of all the various IR frequencies is transmitted to the molecule and the frequencies absorbed are recorded
The absorption frequencies are specified as wavenumbers in units of reciprocal centimeters (cm^-1)

Alternatively the wavelength (λ) in units of microns (μm) can be specified

The spectrum is a plot of frequency on the horizontal axis versus strength of absorption on the vertical axis

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There are different types of stretching and bending vibrations induced by the absorption of infrared energy

The actual relative frequency of vibration can be predicted

Bonds with lighter atoms vibrate faster than those with heavier atoms

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Triple bonds (which are stiffer and stronger) vibrate at higher frequencies than double bonds

Double bonds in turn vibrate at higher frequencies than single bonds

The IR spectrum of a molecule usually contains many peaks

These peaks are due to the various types of vibrations available to each of the different bonds
Additional peaks result from overtone (harmonic) peaks which are weaker and of lower frequency
The IR is a “fingerprint” of the molecule because of the unique and large number of peaks seen for a particular molecule

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Interpreting IR Spectra

Generally only certain peaks are interpreted in the IR

Those peaks that are large and above 1400 cm^-1 are most valuable

Hydrocarbons

The C-H stretching regions from 2800-3300 cm^-1 is characteristic of the type of carbon the hydrogen is attached to
C-H bonds where the carbon has more s character are shorter, stronger and stiffer and thus vibrate at higher frequency

C-H bonds at sp centers appear at 3000-3100 cm^-1
C-H bonds at sp² centers appear at about 3080 cm^-1
C-H bonds at sp³ centers appear at about 2800-3000 cm^-1

C-C bond stretching frequencies are only useful for multiple bonds

C-C double bonds give peaks at 1620-1680 cm^-1
C-C triple bonds give peaks at 2100-2260 cm^-1
These peaks are absent in symmetrical double and triple bonds

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Example: octane

Example: 1- hexyne

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Alkenes

The C-H bending vibration peaks located at 600-1000 cm^-1 can be used to determine the substitution pattern of the double bond

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Example: 1-hexene

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Aromatic Compounds

The C-C bond stretching gives a set of characteristic sharp peaks between 1450-1600 cm^-1
Example: Methyl benzene

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Other Functional Groups

Carbonyl Functional Groups

Generally the carbonyl group gives a strong peak which occurs at 1630-1780 cm^-1

The exact location depends on the actual functional group present

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Alcohols and Phenols

The O-H stretching absorption is very characteristic

In very dilute solutions, hydrogen bonding is absent and there is a very sharp peak at 3590-3650 cm^-1
In more concentrated solutions, the hydroxyl groups hydrogen bond to each other and a very broad and large peak occurs at 3200-3550 cm^-1
A phenol has a hydroxyl group directly bonded to an aromatic ring

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Carboxylic Acids

The carbonyl peak at 1710-1780 cm^-1 is very characteristic
The presence of both carbonyl and O-H stretching peaks is a good proof of the presence of a carboxylic acid

Example: propanic acid

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Amines

Very dilute solution of 1^o and 2^o amines give sharp peaks at 3300-3500 cm^-1for the N-H stretching

1^o amines give two peaks and 2^o amines give one peak
3^o have no N-H bonds and do not absorb in this region

More concentrated solutions of amines have broader peaks
Amides have amine N-H stretching peaks and a carbonyl peak

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