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Organic Compounds and Functional Groups • There are more than 19 million known organic compounds, each with its own physical and chemical properties. • This is very daunting, but organic compounds may be classified into families according to their structures. • Members of the same families tend to have similar properties, so grouping compounds into families helps simplify what would initially seem to be an bewildering number of compounds. • Organic compounds are classified and grouped according to what functional groups they contain. • A functional group is simply a group of atoms which is chemically reactive. • In general, a functional group either contains a π bond or an electronegative or an electropositive atom(s). • So the presence of π bonds or polar bonds is a good indicator of chemical reactivity. • A given functional group will undergo the same kinds of chemical reactions in most molecules in which they are present. • The molecule may be very small and simple, or it may be large, complex, and bulky: but the common functional group will behave in an almost identical manner. Common Functional Groups • The following table contains many of the common functional groups which organic molecules may contain. • Note that in the Table, R is used to signify an alkyl group, a group which contains only sp3 C atoms and H atoms. When more than one R is present in a General Formula, they may or may not be different alkyl groups. Common Functional Groups • What are you responsible for? • You should recognize alkenes, alkyl halides, alcohols, amines, ethers, carboxylic acids, ketones, aldehydes, esters, and amides. Functional Groups with a C-C π Bond • There are several common groups which contain a π C-C bond. • What is a bond? What is a π bond? • Alkenes contain a double bond, while alkynes contain a triple bond. Functional Groups, Cont. • Arenes, where benzene is the simplest example, are 6-membered rings with alternating single and double bonds (this is a simplistic explanation, arenes have resonance structures). • Although you know that double and triple bonds are shorter and stronger than single bonds, the π bond is still chemically active and can undergo certain chemical reactions. Functional Groups with Carbon Singly Bonded to an Electronegative Atom • Carbon is quite commonly bonded to an electronegative atom like O, N, Cl, Br, I, F, or S. • Alkyl halides have C bonded to a halogen; ethers, alcohols have C bonded to O; amines have C bonded to N; and thiols have C bonded to S. • In all of these cases, the end result is a polar bond with the C having a δ+ charge. This partial charge separation leads to chemical reactivity. Carbonyl Functional Groups • There is a common class of compounds where a C atom is double bonded to O, or the C=O group. • It’s generic name is the carbonyl group. • Common carbonyl compounds include amino acids, fatty acids, and soaps. • There are several types of carbonyl compounds as seen in the Table. Carbonyl Functional Groups – carboxylic acids – esters (sweet smelling compounds found in fruits, etc., and used in perfumes) – ketones – aldehydes (usually foul smelling like formaldehyde) – amides (peptides and proteins are amides) Carbonyl Functional Groups – acid halides – acid anhydrides – In all of these groups, the C=O bond is polar with the O atom having a δ- charge and the C atom having a δ+ charge. Properties of Alkanes • Alkanes are hydrocarbons (C, H compounds) which contain only sp3 hybridized C atoms. • Thus, they contain only single bonds. • Because of this, they are also called saturated hydrocarbons, as they are saturated with hydrogen. • They have a CnH2n+2 general formula (except cycloalkanes). Properties of Alkanes • In general, alkanes are not very reactive. • They will react with oxygen, halogens, and a few other substances. • Alkanes are common fuel sources for cars, houses, etc. • Common alkane fuels are methane, propane, butane, and of course think of your octane rating at the gas pump! Properties of Alkanes • When an alkane (or any hydrocarbon) combusts, the two ideal products are carbon dioxide and water, although carbon monoxide and other unwanted products are made as well. • Of course, combustion reactions are exothermic, so a great deal of useful heat is given off as well. • This heat is called the heat of combustion, ΔHcomb. • As you might expect, as you increase the number of C atoms in the alkane, there is more to burn, and more heat is given off. Alkane Halogenation • Alkanes also react with halogens in a reaction called halogenation. • UV light acts as a catalyst in starting the reaction. • One or more halogen atoms may be added to the alkane, so there are a variety of products made. Polarity and Intermolecular Forces • Alkanes are nonpolar molecules as they contain only C-C and C-H single bonds. • So the intermolecular forces acting between molecules are weak London Dispersion Forces. • As you may remember, London Dispersion Forces get stronger as the molar mass or molecule size increases. London Forces & BPt and MPt • alkanes with a carbon chain up to 4 are gases at room temperature • alkanes with a C chain of up to 17 are liquids • alkanes with a C chain of over 17 are solids. London Forces & Molecular Shape • You may also remember, that the bulkiness or shape of the molecule also affects the strength of the London Dispersion Forces. • The closer 2 or more molecules may come together, the stronger the London Forces will be, so the higher the boiling and melting point. • So if a molecule is very bulky with lots of alkyl group branching, molecules can’t get as close together. • Continuous-chain hydrocarbons have higher boiling and melting points than do what we call branched-chain alkanes. Alkane Solubilities • Because alkanes are nonpolar, they are not very soluble in water or other very polar solvents. • Instead, they are soluble in relatively nonpolar solvents like benzene, ethers, etc. • The densities of alkanes are lower than water, which you should know as oil and gas float on water! Cycloalkanes • Cycloalkanes are ring structure where there are no end C; or you could say that the two end C have joined to form a ring. • Because of this extra C-C bond, 2 H atoms are lost, so the general formula is CnH2n. • But, just like alkanes, they contain only sp3 hybridized C atoms. Properties of Cycloalkanes • The properties of cycloalkanes are very similar to those of alkanes. • However, if you use a molecular model kit and make some cycloalkanes, you will notice a few differences. • Cycloalkanes, particularly small cycloalkanes, are highly symmetric. Properties of Cycloalkanes • You should also notice that cycloalkanes do not rotate around C-C bonds as freely as regular alkanes. • In open-chain alkanes, there is free rotation around all of the C-C bonds. • But in cycloalkanes, this C-C rotation is greatly hindered by the closed ring: if there were free rotation around the C-C bond, the ring might be broken. Properties of Cycloalkanes • The smallest cycloalkanes, C3 to C6 or C7, have rigid rings with little C-C rotation possible. • Because of this rigidity, these rings are strained, and they are more reactive than larger rings. • As cycloalkanes get larger, the rigidity of the ring decreases, the ring strain decreases, and there is more freedom of rotation. • Large cycloalkanes are floppy, and are virtually identical to open-chain molecules. Properties of Cycloalkanes • Because of their greater symmetry and hindered C-C rotation, cycloalkanes generally have higher melting points, boiling points, and densities than openchain alkanes. • See the following Table for examples. Compound B.Pt. M.Pt. Pentane (C5H12) 36.1 -129.8 Density (g/mL) 0.5582 Cyclopentane 49.3 (C5H10) Octane 125.7 (C8H18) -93.9 0.7457 -56.8 0.7026 Cyclooctane (C8H16) 14.3 0.8349 150.0