Functional Group Ranking by Boiling Points

Functional Group Ranking by Boiling Points R = any number carbons in a hydrocarbon chain

Boiling Point

Polar Rank (most to least)

Name

Amide

222o

1

ethanamide

Acid

118o

2

ethanoic acid or acetic acid

(2) ACID: These compounds are second in the polarity because of hydrogen bonding capabilities and the presence of two oxygen atoms.

Alcohol

117o

3

propanol

(3) ALCOHOL: These compounds are third in the polarity because of hydrogen bonding capabilities and the presence of only one oxygen vs. the two in the acid functional group.

Functional Group Name

Ketone

56o

4, 5

propanone or acetone

Aldehyde

49o

4, 5

propanal

Amine

49

o

6

propylamine

Ester

32o

7

methyl ethanoate

Ether

11o

8

methyl ethyl ether

Alkane

-42o

9

propane

Brief Explanation

(1) AMIDE: Perhaps it is surprising that the amide appears to be the most polar according to the data. The reason is that it can both hydrogen bond and accept hydrogen bonds on both the oxygen and the nitrogen.

(4) KETONE and (5) ALDEHYDE: A comparison of the boiling points of aldehyde and ketone with the corresponding alcohol shows that the alcohol is more polar due to its ability to hydrogen bond. Since ketones and aldehydes lack hydroxyl groups, they are incapable of intermolecular hydrogen bonds. But due to the presence of the oxygen, they can accept hydrogen bonds from water molecules which account for the complete solubility of low molecular weight compounds. On the other hand, their boiling points are considerable higher than the ether or alkane, indicating the presence of weak intermolecular dipoledipole forces. The carbonyl group ("carbon double bond oxygen") is polar since oxygen is more electronegative than carbon and forms a partially charged dipole. (6) AMINE : The polarity of the amine nitrogen is shown to be much less than the oxygen in alcohol group. The nitrogen in the amine is much less electronegative than oxygen in the alcohol. Therefore, the dipole on N-H is much weaker than the dipole on O-H. (7) ESTER: The ester functional group has a similar character to the ketone and aldehyde functional group. The boiling point indicates that it is the least polar of the three. 8) ETHER : The carbon-oxygen-carbon bond in ethers is much like the carbon-carbon bond in alkanes. The lack of any oxygen-hydrogen bond makes hydrogen bonding impossible. There is very little intermolecular association. Therefore, the properties of ethers are much like alkanes. Ethers are essentially non-polar and insoluble in water. (9) HYDROCARBON: There is very little intermolecular association because the carbon-hydrogen bond is non-polar. Alkanes, alkenes, and alkynes are essentially non-polar and insoluble in water.

Polarity of Organic Compounds Principles of Polarity:The greater the electronegativity difference between atoms in a bond, the more polar the bond. Partial negative charges are found on the most electronegative atoms, the others are partially positive. In general, the presence of an oxygen is more polar than a nitrogen because oxygen is more electronegative than nitrogen. The combination of carbons and hydrogens as in hydrocarbons or in the hydrocarbon portion of a molecule with a functional group is always NON-POLAR. Polarity Ranking of the Functional Groups: (most polar first) Amide > Acid > Alcohol > Ketone ~ Aldehyde > Amine > Ester > Ether > Alkane An abbreviated list to know well is: Amide > Acid > Alcohol > Amine > Ether > Alkane Boiling Point Definition:In a liquid the molecules are packed closely together with many random movements possible as molecules slip past each other. As a liquid is heated, the temperature is increased. As the temperature increases, the kinetic energy increases which causes increasing molecular motion (vibrations and molecules slipping pas each other). Eventually the molecular motion becomes so intense that the forces of attraction between the molecules is disrupted to to the extent the molecules break free of the liquid and become a gas. At the temperature of the boiling point, the liquid turns into a gas. The molecules are not in contact each other in the gaseous state. Polarity and Boiling Point:The polarity of the molecules determines the forces of attraction between the molecules in the liquid state. Polar molecules are attracted by the opposite charge effect (the positive end of one molecule is attracted to the negative end of another molecule. Molecules have different degrees of polarity as determined by the functional group present. Principle: The greater the forces of attraction the higher the boiling point or the greater the polarity the higher the boiling point.

1. The relative strength of the four intermolecular forces is: Ionic > Hydrogen bonding > dipole dipole > Van der Waals dispersion forces. The influence of each of these attractive forces will depend on the functional groups present.

2. Boiling points increase as the number of carbons is increased. 3. Branching decreases boiling point.

Because of their greater molecular weights, alkyl halides have considerably higher boiling points than unhalogenated alkanes with the same number of carbons. For a given alkyl group, the boiling point increases with increasing atomic weight of the given halogen, so that a fluoride is the lowest boiling and an iodide the highest. For a given halogen, the boiling point rises with increasing carbon number. Branching of any kind (R group or halogen X) lowers the boiling point. In spite of their modest polarity, alkyl halides are insoluble in water, probably because of their inability to form hydrogen bonds. They are soluble in the typical organic solvents of low polarity, like benzene, ether, chloroform, or ligroin (petroleum ether). Iodo, bromo, and polychloro compounds are more dense than water ( > 1.0 g / cc). Thus, as molecules of low polarity, both the alkanes and alkyl halides are held together by van der Waals forces or weak dipole-dipole attractions. The are characterized by low melting points and boiling points, and are soluble in non-polar solvents. Boiling points and solubility Thiols show little association by hydrogen bonding, with both water molecules and among themselves. Hence, they have lower boiling points and are less soluble in water and other polar solvents than alcohols of similar molecular weight. Thiols and thioethers have similar solubility characteristics and boiling points. Alkenes contains a carbon-carbon double bond. This carbon-carbon double bond changes the physicals properties of alkenes. At room temperatue, alkenes exist in all three phases, solid, liquids, and gases. Melting and boiling points of alkenes are similar to that of alkanes, however, isomers of cis alkenes have lower melting points than that of trans isomers. Alkenes display a weak dipole-dipole interactions due to the electron-attracting sp2 carbon. Due to their low polarity, the alkynes have physical properties that are essentially the same as those of the alkanes and alkenes. They are insoluble in water, but quite soluble in the usual organic solvents of low polarity: ligroin, ether, benzene, and carbon tetrachloride. They are less dense than water, with relative densities ranging from 0.67 to 0.77. Their boiling points show the usual increase with increasing carbon number, and the usual effects of chain-branching (lower melting and boiling points due to increased surface area). Unlike alkanes, alkynes are unstable and very reactive. This gives rise to the intense heat (> 3000 °C) of the acetylene flame used in welding.