Which of the following bonds is essentially nonpolar?

We have mentioned the hydrophobic nature of the many C—C and C—H bonds in a fatty acid. But what about the carboxyl functional group at the end of the molecule? When it ionizes and forms COO−, it is strongly hydrophilic. So a fatty acid has two opposing chemical properties: a hydrophilic end and a long hydrophobic tail. A molecule that is partly hydrophilic and partly hydrophobic is said to be amphipathic.

In triglycerides, a glycerol molecule is bonded to three fatty acid chains and the resulting molecule is entirely hydrophobic. Phospholipids are like triglycerides in that they contain fatty acids bound to glycerol. However, in phospholipids, a phosphate-containing compound replaces one of the fatty acids, giving these molecules amphipathic properties (FIGURE 2.13A). The phosphate functional group (there are several different kinds in different phospholipids) has a negative electric charge, so this portion of the molecule is hydrophilic, attracting polar water molecules. But the two fatty acids are hydrophobic, so they tend to avoid water and aggregate together or with other hydrophobic substances.

Figure 2.13: Phospholipids (A) Phosphatidylcholine (lecithin) is an example of a phospholipid molecule. In other phospholipids, the amino acid serine, the sugar alcohol inositol, or another compound replaces choline. (B) In an aqueous environment, hydrophobic interactions bring the “tails” of phospholipids together in the interior of a bilayer. The hydrophilic “heads” face outward on both sides of the bilayer, where they interact with the surrounding water molecules.

In an aqueous environment, phospholipids line up in such a way that the nonpolar, hydrophobic “tails” pack tightly together and the phosphate-containing “heads” face outward, where they interact with water. The phospholipids thus form a bilayer: a sheet two molecules thick, with water excluded from the core (FIGURE 2.13B). Although no covalent bonds link individual lipids in these large aggregations, such stable aggregations form readily in aqueous conditions. Biological membranes have this kind of phospholipid bilayer structure, and we will devote Chapter 5 to their biological functions.

CHECKpoint CONCEPT 2.4

• What is the difference between fats and oils?
• Why are phospholipids amphipathic, and how does this result in a lipid bilayer membrane?
• If fatty acids are carefully put onto the surface of water, they form a single molecular layer. If the mixture is then shaken vigorously, the fatty acids will form round structures called micelles. Explain these observations.

Molecules such as carbohydrates and lipids are not always stable in living systems. Rather, a hallmark of life is its ability to transform molecules. This involves making and breaking covalent bonds, as some atoms are removed and others are attached. As part of our introduction to biochemical concepts, we will now turn to these processes of chemical change.

If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

In a diatomic molecule (X2 or XY), there is only one bond, and the polarity of that bond determines the polarity of the molecule: if the bond is polar, the molecule is polar, and if the bond is nonpolar, the molecule is nonpolar.

In molecules with more than one bond, both shape and bond polarity determine whether or not the molecule is polar. A molecule must contain polar bonds in order for the molecule to be polar, but if the polar bonds are aligned exactly opposite to each other, or if they are sufficiently symmetric, the bond polarities cancel out, making the molecule nonpolar. (Polarity is a vector quantity, so both the magnitude and the direction must be taken into account.)

For example, consider the Lewis dot structure for carbon dioxide. This is a linear molecule, containing two polar carbon-oxygen double bonds. However, since the polar bonds are pointing exactly 180� away from each other, the bond polarities cancel out, and the molecule is nonpolar. (As an analogy, you can think of this is being like a game of tug of war between two teams that are pulling on a rope equally hard.)

The water molecule also contains polar bonds, but since it is a bent molecule, the bonds are at an angle to each other of about 105�. They do not cancel out because they are not pointing exactly towards each other, and there is an overall dipole going from the hydrogen end of the molecule towards the oxygen end of the molecule; water is therefore a polar molecule:

Molecules in which all of the atoms surrounding the central atom are the same tend to be nonpolar if there are no lone pairs on the central atom. If some of the atoms surrounding the central atom are different, however, the molecule may be polar. For example, carbon tetrachloride, CCl4, is nonpolar, but chloroform, CHCl3, and methyl chloride, CH3Cl are polar:

The polarity of a molecule has a strong effect on its physical properties. Molecules which are more polar have stronger intermolecular forces between them, and have, in general, higher boiling points (as well as other different physical properties).

The table below shows whether the examples in the previous sections are polar or nonpolar. For species which have an overall charge, the term �charged� is used instead, since the terms �polar� and �nonpolar� do not really apply to charged species; charged species are, by definition, essentially polar. Lone pairs on some outer atoms have been omitted for clarity.

Formula

Lewis Structure

3D Structure

Shape

 Polarity

Explanation

1.

CH4

tetrahedral

nonpolar

The C�H bond is nonpolar, since C and H differ by only 0.35 electronegativity units.

2.

NH3

trigonal pyramidal

polar

Since this molecule is not flat, the N�H bonds are not pointing directly at each other, and their polarities do not cancel out. In addition, there is a slight dipole in the direction of the lone pair.

3.

H2O

bent

polar

Since this molecule is bent, the O�H bonds are not pointing directly at each other, and their polarities do not cancel out.

4.

H3O+

trigonal pyramidal

charged

Since this species is charged, the terms �polar� and �nonpolar� are irrelevant.

5.

HCN

linear

polar

Linear molecules are usually nonpolar, but in this case, not all of the atoms connected to the central atom are the same. The C�N bond is polar, and is not canceled out by the nonpolar C�H bond.

6.

CO2

linear

nonpolar

The polar C=O bonds are oriented 180� away from each other. The polarity of these bonds cancels out, making the molecule nonpolar.

7.

CCl4

tetrahedral

nonpolar

The polar C�Cl bonds are oriented 109.5� away from each other. The polarity of these bonds cancels out, making the molecule nonpolar.

8.

COCl2

trigonal planar

polar

Trigonal planar molecules are usually nonpolar, but in this case, not all of the atoms connected to the central atom are the same. The bond polarities do not completely cancel out, and the molecule is polar. (If there were three O�s, or three Cl�s attached to the central C, it would be nonpolar.)

9.

O3

bent

polar

Bent molecules are always polar. Although the oxygen-oxygen bonds are nonpolar, the lone pair on the central O contributes some polarity to the molecule.

10.

CO32-

trigonal planar

charged

Since this species is charged, the terms �polar� and �nonpolar� are irrelevant.

11.

C2H6

tetrahedral

nonpolar

Both carbon atoms are tetrahedral; since the C�H bonds and the C�C bond are nonpolar, the molecule is nonpolar.

12.

C2H4

trigonal planar

nonpolar

Both carbon atoms are trigonal planar; since the C�H bonds and the C�C bond are nonpolar, the molecule is nonpolar.

13.

CH3CH2OH

C: tetrahedral

O: bent

polar

The C�C and C�H bonds do not contribute to the polarity of the molecule, but the C�O and O�H bonds are polar, the since the shape around the O atom is bent, the molecule must be polar.

14.

BF3

trigonal planar

nonpolar

Since this molecule is planar, all three polar B�F bonds are in the same plane, oriented 120� away from each other, making the molecule nonpolar.

15.

NO

linear

polar

Since there is only one bond in this molecular, and the bond is polar, the molecule must be polar.

16.

PCl5

trigonal bipyramidal

nonpolar

The P�Cl bonds in the equatorial positions on this molecule are oriented 120� away from each other, and their bond polarities cancel out. The P�Cl bonds in the axial positions are 180� away from each other, and their bond polarities cancel out as well.

17.

SF6

octahedral

nonpolar

The S�F bonds in this molecules are all 90� away from each other, and their bond polarities cancel out.

18.

SF4

seesaw

polar

The S�F bonds in the axial positions are 90� apart, and their bond polarities cancel out. In the equatorial positions, since one position is taken up by a lone pair, they do not cancel out, and the molecule is polar.

19.

XeF4

square planar

nonpolar

The Xe�F bonds are all oriented 90� away from each other, and their bond polarities cancel out. The lone pairs are 180� away from each other, and their slight polarities cancel out as well.

Is a triglyceride polar or nonpolar?

Which of the following is are lipids?

Is starch considered a lipid?

Which of the following is a characteristic that all lipids share?