Hydrocarbons Functional Groups Chemistry 11 Ms. McGrath

Functional Groups A functional group is a reactive group of bonded atoms that appears in all the members of a chemical family. The functional group is the reactive part of an organic molecule. Each functional group behaves and reacts in a characteristic way. Thus, functional groups help to determine the physical and chemical properties of compounds. For example, the reactive double bond is the functional group for an alkene.

Functional Groups Common Functional Groups – see page 378 Note the functional groups Organic compounds are named according to their functional group We have seen alkanes, alkenes, alkynes, cyclic hydrocarbons and aromatics (benzene) We are going to look at alcohols, aldehydes, ketones, ether

Functional Groups 1. Compounds with the same functional group often have similar physical properties 2. Compounds with the same functional group react chemically in very similar ways

Functional Groups Functional groups usually contain polar bonds Examples of these bonds are O-H, C-O, C=O Many hydrocarbons contain C-H bonds which are weak dipoles. The several C-H bonds partially cancel each other out. There C-H bonds contribute very little to the polarity of a hydrocarbon – they are neglected when considering the polarity of the molecule

Physical Properties Substituted Hydrocarbons Recall from Chapter 6, we learned about 3 intermolecular forces: 1. Hydrogen Bonds 2. Dipole Interactions 3. Dispersion Forces ** Note melting points of organic compounds follow approximately the same trend as do their boiling points **

Physical Properties Hydrogen Bonds Molecules that O-H, N-H and F-H bonds, they can form hydrogen bonds with themselves and water These molecules have higher boiling points than similar molecules that cannot form hydrogen bonds For example, alcohols that have an O-H bond can form hydrogen bonds but alkanes cannot Shorter chained molecules that can form hydrogen bonds are soluble in water

Physical Properties Dipole Interactions Polar molecules usually have a higher boiling point than non-polar molecules of a similar size but not as high as molecules than can form hydrogen bonds For example, ethanol, CH3CH2OH, is polar and can form hydrogen bonds. Methoxymethane, CH3OCH3, is also polar but cannot form hydrogen bonds. Ethane, CH3CH3, is non-polar and cannot form hydrogen bonds. Ethanol has the highest boiling point, then methoxymethane and ethane has the lowest.

Physical Properties Dispersion Forces These are weak forces A molecule with a greater number of carbon atoms usually has a higher boiling point than the same type of molecule with fewer carbon atoms For example, hexane, CH3CH2CH2CH2CH2CH3, has a higher boiling point than ethane, CH3CH3

Physical Properties Complete ThoughtLab – Comparing Intermolecular Forces Page 380

Functional Group Alcohol General Formula R-OH Where: R represents an alkyl group OH represents an hydroxyl group (the functional group of the alcohol family, written as –OH)

Alcohol