Functional Groups

What are functional groups? Remember back to alkenes and alkynes A group of bonded atoms that appear in all members of a chemical family The functional group results in similar properties for all members of the group. Ex. 2 – propanol (isopropanol) and ethanol have very similar properties due to the (-OH) hydroxyl group.

General Formulas General formulas for organic compounds emphasize the functional group. Why? Functional groups are most important. How? R- stands for the alkyl group(part) of the molecule If more than one type alkyl group is present we use R’ and R” to tell show that they are different

General Formulas Eg. The general formula for alcohols is: R-OH, where R- represents the hydrocarbon or alkyl part Therefore, R-OH represents ALL alcohols CH3OH CH3CH2OH CH3CH2CH2OH, etc.

Naming Organic Compounds According to functional group This determines properties Suffix – indicates the most important functional group Eg. Ethene = double bond Ethanol = hydroxyl group

Physical Properties and Forces Between Molecules

Physical Properties of Organic Compounds Melting Point Boiling Point Solubility All depend on functional groups Same functional group = similar properties Why? Physical properties are determined by intermolecular forces = forces b/n molecules

Intermolecular Forces 3 types play a role Hydrogen bonding Dipole-dipole interactions Dispersion forces Decreasing Strength

Hydrogen Bonding Attraction between the H atom from an N-H, O-H or F-H group on one molecule and a N, O or F atom on another molecule. E.g. Hydrogen bonding in H2O

Dipole-dipole Interactions Attractive force b/n polar molecules. Cause polar molecules to stick together. Weaker than H bonding b/c electronegativity difference is less within the covalent bond. E.g.

Dispersion Forces (London or Van der Waals Forces) Weak attractive force Occurs between all covalent molecules as the size of the molecule  How are they created? Random, uneven electron distribution occurs within molecules that come close to one another. Weak attraction b/n nucleus of one molecule and electrons of the other.

Predicting Properties based Intermolecular Forces

Can the molecules form hydrogen bonds? Must have O-H, N-H, H-F Can form H-bonds with themselves and H2O Molecules can form H-bonds with themselves = higher boiling point If they can form H-bonds with water they will usually be soluble in water.

Are the molecules polar? They are if: They have polar bonds The polar bonds do not counter act each other ie. They act in opposite directions

Polar Molecules Are attracted to each other by dipole-dipole forces Have lower boiling points than similar molecules with hydrogen bonding but higher than non-polar molecules. As non-polar part of a molecule gets larger  solubility decreases. Eg. CH3CH2OH is more soluble than CH3CH2CH2CH2OH

Relative boiling points CH3CH2OH – ethanol (hydrogen bonds, dipole-dipole and dispersion) CH3OCH3 – methyoxymethane (dipole-dipole and dispersion) CH3CH3 – ethane (dispersion forces) Why? Molecules need more kinetic energy to break the intermolecular forces. Boiling Point

How strong are the dispersion forces? Weakest of the 3 Stronger as the hydrocarbon part of the molecule gets larger Therefore longer molecules tend to have higher boiling points Eg. ethane (-89°C) hexane (69°C)