Unit 1 – Organic Chemistry The properties and characteristics of organic compounds

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.

Intermolecular forces 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.

Intermolecular forces 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

Intermolecular forces The polarity of a bond is determined by the electronegativity difference. Electronegativity Difference (DE) Non-polar Polar Ionic 3.3 1.7 0.4 Organic compounds

Intermolecular forces 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.

Intermolecular forces 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

Intermolecular forces 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

Intermolecular forces 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. DE difference) of the polar bonds Number and placement of the polar bonds Size of the molecules

Intermolecular forces 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

Assessing intermolecular forces 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.

Assessing intermolecular forces 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 increases 1-5 C’s  gases @ SATP 5-20 C’s  liquids (some very viscous) @SATP 20 + 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 Polarity of functional group 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 Melting & Boiling Points Due to the strength of hydrogen bonds most alcohols have higher melting & boiling points than similar alkanes. Most alcohols are liquids at SATP

Alcohols Additional 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 Polarity of functional group 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 Melting & Boiling Points Much lower than comparable alcohols Higher than similar hydrocarbons

Ethers Additional 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 Polarity of functional group 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 Melting & Boiling Points The boiling points of primary and secondary amines (N-H bonds present) are greater than tertiary amines (no N-H bonds)

Amines Additional 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 Polarity of functional group 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 Melting & Boiling Points Lower than comparable alcohols Higher than corresponding alkanes

Aldehydes & Ketones Additional 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 Polarity of functional group 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 Melting & Boiling Points The melting and boiling points for carboxylic acids are very high due to the hydrogen bonding. Higher than corresponding alcohols

Carboxylic acids Additional 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 CH3CH2CH2COH = b.p. 165.5 oC m.p. -4.5 oC Carboxylic acid High b.p. 117.2 oC m.p. -89.5 oC Alcohol CH3CH2CH2CH2OH CH3CH2CCH3 = b.p. 79.6 oC m.p. -86.3 oC Ketone CH3CH2CH2CH = b.p. 75.7 oC m.p. -99 oC Aldehyde b.p. -0.5 oC m.p. -138.4 oC Alkane CH3CH2CH2CH3 Low

Esters (R-COO-R2) Physical Properties Polarity of functional group 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 Melting & Boiling Points The melting and boiling points for esters are low due to the lack of hydrogen bonding. They are usually volatile liquids at SATP

Esters Additional 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 Polarity of functional group 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 Melting & Boiling Points Primary amides have much higher melting and boiling points than carboxylic acids Many primary amides are solids at SATP

Amides Additional Characteristics Amides such as acetaminophen are analgesics (pain killers) Urea is a common amide found in urine and fertilizers