the properties and characteristics of organic compounds unit 1 – organic chemistry

The properties and characteristics

of organic compounds

Unit 1 – Organic Chemistry

Intermolecular forces

The physical and chemical properties are determined by the bonds and forces between atoms and molecules. Intramolecular forces occur between

atoms in molecules and determine a substance’s chemical properties.

Intermolecular forces occur between molecules and affect a substance’s physical properties.


The physical properties of organic compounds are affected by the intermolecular forces of attraction and repulsion between molecules

Larger forces of intermolecular attraction result in high boiling & melting points.

Weak intermolecular forces result in low boiling & melting points producing gas states.


The foundation for these intermolecular forces lies in the types of bonds that form between atoms in molecules.

The bonding continuum ranges from non-polar to ionic, however organic molecules are predominantly covalent molecules and lie in the polar to non-polar range.

Polar and non-polar regions affect solubility


The polarity of a bond is determined by the electronegativity difference.

Non-polar Polar Ionic

3.31.70.40

Electronegativity Difference (E)

Organic compounds


The intermolecular forces are attractive and repulsive forces between molecules determined by factors such as: The bond types within the molecules.

Polar vs. Non-polar The size of the molecules. The shape of the molecules.


Johannes van der Waals (1837-1923) studied these forces and they are often referred to by his name.

We will focus on the three main types: Dispersion (London) forces Dipole-dipole forces Hydrogen bonding


Dispersion (London) forces The constant vibration of electrons in

covalent bonds generates temporary regions of charge distribution and polar regions which result in weak forces of attraction between all covalent molecules.

The overall strength of these forces is proportional to; The number of electrons in the molecule The size and shape of the molecule


Dipole-Dipole Forces Polar molecules orient themselves so that

electrostatic attraction occurs between the different charged regions of the molecules.

These forces are generally stronger than the dispersion (London) forces.

Their strength depends upon the; Nature (i.e. difference) of the polar bonds Number and placement of the polar bonds Size of the molecules


Hydrogen bonding A strong form of dipole-dipole attraction between

a hydrogen atom and oxygen, nitrogen or fluorine in a polar covalent molecule.

The high electronegative difference results in a strong polar bond and distinct regions of charge distribution.

Hydrogen bonding is responsible for water’s unique characteristics and influences organic compounds’ solubility in water.

Assessing intermolecular forces

Can molecules form hydrogen bonds? If so:

They have higher boiling and melting points than similar molecules that cannot form hydrogen bonds

They form hydrogen bonds with water and thereby have greater solubility in water


Are they polar? Polar molecules tend to have higher boiling &

melting points than similar non-polar ones. Hydrogen bonding capability increases the trend further.

A large non-polar region (hydrocarbon) in a molecule reduces the affect of the smaller polar regions and are less soluble than smaller hydrocarbon polar molecules.


How strong are the dispersion forces? Large hydrocarbon chains have stronger

dispersion forces. The boiling and melting points tend to be

proportional to the number of carbons in the hydrocarbon. Large hydrocarbons have high boiling and melting

points. Small hydrocarbons tend to be gases at SATP.

Hydrocarbons (CnH2n+2) Physical Properties

Polarity of functional group

The C-H bond is non-polar so most hydrocarbons are non-polar

The addition of halides results in polar regions. The predominant intermolecular force in dispersion

forces

Hydrogen bonding

None

Solubility in Water

Very poor solubility in water due to the lack of polar and hydrogen bonds

Melting & Boiling Points

The melting and boiling points tend to be very low The boiling points increase as the length of the parent

chain increases1-5 C’s gases @ SATP5-20 C’s liquids (some very viscous) @SATP20 + C’s solids @ SATP

Hydrocarbons (CnH2n+2)Additional Characteristics

As the number of carbons in the parent chain increase: The state of matter at SATP moves from

gas to liquid to solid The melting and boiling points increase The density increases

Popular fuel source (Methane, propane, octane, etc.)

Source for plastics

Alcohols (R-OH)Physical Properties


OH is very polar As the parent chain grows, its non-polar nature become

more influential Small alcohols are more polar than larger ones

Hydrogen bonding

Alcohols experience hydrogen bonding with other alcohols and water

Solubility in Water

The capacity to form hydrogen bonds make alcohols extremely soluble in water.

The solubility decreases as the number of carbons increase


Due to the strength of hydrogen bonds most alcohols have higher melting & boiling points than similar alkanes.

Most alcohols are liquids at SATP

AlcoholsAdditional Characteristics

Alcohols are extremely flammable Most alcohols are poisonous

Methanol can cause blindness or death. Ethanol is consumed in moderate quantities.

Excessive consumption may cause blindness or death.

Yeast ferment sugars to produce ethanol and carbon dioxide

Antifreeze, cosmetics, foods, medical preparations, solvents, antiseptics, etc.

Biological compounds – cholesterol, retinol, etc.

Ethers (R-O-R2)Physical Properties


The C-O bond is less polar than the O-H Ethers are less polar than alcohols The bent shape around the O aids in the dipole-

dipole force strength

Hydrogen bonding

There is no hydrogen bonding in ethers Ethers can accept hydrogen bonds from water

Solubility in Water

Ethers are usually soluble in water The solubility decreases as the size of the alkyl

groups grows


Much lower than comparable alcohols Higher than similar hydrocarbons

EthersAdditional Characteristics

Ethers are extremely flammable Have been used as anaesthetics

“ether” – ethoxyethane More recent anaesthetic

(1-methoxypropane) has fewer side effects.

Amines (R-NH2)Physical Properties


C-N and N-H bonds are polar Amines are usually polar

Hydrogen bonding

The presence of one or more N-H bonds allows hydrogen bonding

Solubility in Water

Low molecular weight amines are soluble in water (<4 C) The solubility decreases as the number of carbons

increase


The boiling points of primary and secondary amines (N-H bonds present) are greater than tertiary amines (no N-H bonds)

AminesAdditional Characteristics

Widely found in nature. They are often toxic. Many medical applications.

Low molecular weight amines have a “fishy” smell while others are responsible for odours associated with decay.

Amines act as weak bases. Adding an acid produces a salt. So, add lemon to fish to remove fishy odour.

Aldehydes (R-COH) & Ketones Physical Properties


C=O is polar

Hydrogen bonding

No O-H bonds so no hydrogen bonding between aldehydes and ketones

The O atom can accept hydrogen bonds from water

Solubility in Water

Low molecular weight aldehydes and ketones have high solubility

The solubility decreases as the number of carbons increase


Lower than comparable alcohols Higher than corresponding alkanes

Aldehydes & KetonesAdditional Characteristics

Aldehydes have a strong pungent odour while ketones smell sweet. As the size of the aldehyde increases the odour becomes more pleasant (i.e. cinnamon) Odours in nature may be used to communicate between

insects - pheromones Since they are polar, they are used as polar solvents.

The non-polar alkyl chain permits their use as non-polar solvents too.

Antiseptics, disinfectants (Formaldehyde), resins, dyes, preservatives (Aldehyde)

Hormones in biology – testosterone, progesterone, cortisone

Solvents, medication

Carboxylic acids (R-COOH)Physical Properties


Polar due to C=O and O-H bonds

Hydrogen bonding

Strong hydrogen bonding

Solubility in Water

Low molecular masses are very soluble (<4C – miscible in water)

The solubility decreases as the number of C’s increase


The melting and boiling points for carboxylic acids are very high due to the hydrogen bonding.

Higher than corresponding alcohols

Carboxylic acidsAdditional Characteristics

Carboxylic acids often have unpleasant odours butanoic acid smells like stale sweat

Sour taste in many foods & plants – vinegar, spoiled wine, rhubarb, citrus fruits

By-product of anaerobic respiration in muscles – lactic acid

Industrial processes – dyes, tanning, rubber recycling, etc.

Food preservatives, Vitamin C, ASA The OH does not behave like a base. The two O’s

have high electronegativities and carry the H’s electron allowing the H ion to dissociate.

Boiling & Melting Points

CH3CH2CH2CH3

CH3CH2CH2CH

0=

CH3CH2CCH3

0=

CH3CH2CH2CH2OH

CH3CH2CH2COH

0=

Alkane

Aldehyde

Ketone

Alcohol

Carboxylic acidb.p. 165.5 oCm.p. -4.5 oC

b.p. 117.2 oCm.p. -89.5 oC

b.p. 79.6 oCm.p. -86.3 oC

b.p. 75.7 oCm.p. -99 oC

b.p. -0.5 oCm.p. -138.4 oC Low

High

Esters (R-COO-R2)Physical Properties


Usually polar due to C=O and C-O bonds Polarity is reduced with increased number of C atoms

Hydrogen bonding

No O-H bonds, there fore they don’t form hydrogen bonds with other esters

Solubility in Water

The C=O bond can accept hydrogen bonds from water, therefore low molecular mass esters are soluble

Esters with chains greater than 3 or 4 are not soluble


The melting and boiling points for esters are low due to the lack of hydrogen bonding.

They are usually volatile liquids at SATP

EstersAdditional Characteristics

Esters often have pleasant odours Low molecular mass esters have very

pleasant odours and are used in soaps, perfumes, cosmetics, air fresheners, etc.

Natural esters are responsible for the scents in flowers & fruits. (Pg 64 Table 2)

Food – lipids and dietary fats are esters (triglycerides)

Amides (R-CONHR2)Physical Properties


Polar due to C=O,C-N and N-H bonds Similar physical properties to carboxylic acids

Hydrogen bonding

Since primary amides have two N-H bonds they have stronger hydrogen bonding than carboxylic acids

Secondary amides also experience hydrogen bonding

Solubility in Water

Amides are soluble in water The solubility decreases as the non-polar alkyl chain

increases in size


Primary amides have much higher melting and boiling points than carboxylic acids

Many primary amides are solids at SATP

AmidesAdditional Characteristics

Amides such as acetaminophen are analgesics (pain killers)

Urea is a common amide found in urine and fertilizers

the properties and characteristics of organic compounds unit 1 – organic chemistry

Documents

organic molecules

covalent molecules

weak intermolecular

similar molecules

nonpolar regions

polar bonds size

polar bondsnumber

repulsive forces