Organic Functional Groups

1. Organic Functional Groups

2. Functional Groups • A functional group is an atom or a group of atoms with characteristic chemical and physical properties. It is the reactive part of the molecule. • Most organic compounds have C—C and C—H bonds. However, many organic molecules possess other structural features: • Heteroatoms—atoms other than carbon or hydrogen. •  Bonds—the most common  bonds occur in C—C and C—O double bonds. • These structural features distinguish one organic molecule from another. They determine a molecule’s geometry, physical properties, and reactivity, and comprise what is called a functional group.

3. Hydrocarbons are compounds made up of only the elements carbon and hydrogen. They may be aliphatic or aromatic.

4. Aromatic Groups • Aromatic hydrocarbons are so named because they have strong characteristic odors. • The simplest aromatic hydrocarbon is benzene. The six-membered ring and three  bonds of benzene comprise a single functional group.

5. Functional Groups

6. Functional Groups • Ethane: This molecule has only C—C and C—H bonds, so it has no functional group. Ethane has no polar bonds, no lone pairs, and no  bonds. Consequently, ethane and molecules like it are relatively unreactive. • Ethanol: This molecule has an OH group attached to its backbone. This functional group is called a hydroxy group. Ethanol has lone pairs and polar bonds that make it reactive with a variety of reagents. • The hydroxy group makes the properties of ethanol very different from the properties of ethane.

7. Alkyl Groups • Alkyls are alkane side chains or alkane like substituents on an organic molecule • Alkyls are nonpolar groups • Alkyls: Methyl -CH3 (Me-) Ethyl -CH2CH3 (Et-) n-butyl -(CH2)3CH3 (nBu-)

8. Functional Groups on unsaturated aliphatic hydrocarbons Vinylic substituentsAllylic substituents • Enol Allylic alcohol • Enol ether Allylic ether • Enamine R = Alkyl

9. Functional Groups on Aromatic hydrocarbons • Phenol • Aniline

10. Examples of Molecules Containing C-O  Bonds Polycyclic ether

11. Carbonyls • Compounds Containing the C=O Group: • This group is called a “carbonyl group”. • The carbonyl group contains a polar C—O  bond and a C-O  bond that is more easily broken than a C—O  bond.

12. Carbonyls (R)2C=O

13. Functional groups in Biomolecules • Biomolecules are organic compounds found in biological systems. • There are four main families of small molecule biomolecules: • amino acids, simple sugars, lipids and nucleotides • Biomolecules often have several functional groups.

14. Molecules Containing Multiple Functional Group

15. Functional Groups It should be noted that the importance of a functional group cannot be overstated. A functional group determines all of the following properties of a molecule: bonding and shape chemical reactivity type and strength of intermolecular forces physical properties nomenclature Bioactivity

16. Reactivity with Unsaturated Functional Groups • Heteroatoms and  bonds confer reactivity on a particular molecule. • Heteroatoms have lone pairs and create electron-deficient sites on carbon. •  Bonds are easily broken in chemical reactions. A  bond makes a molecule a base and a nucleophile.

17. Influence of Functional Groups on Reactivity • Recall that: • Functional groups create reactive sites in molecules. • Electron-rich sites react with electron poor sites. • All functional groups contain a heteroatom, a  bond or both, and these features create electron-deficient (or electrophilic) sites and electron-rich (or nucleophilic) sites in a molecule. Molecules react at these sites.

18. Nucleophiles (Nu-) or (Nu:) Lewis Bases can act as Nucleophiles

19. Alkene (Lewis base) reacting with an Electrophile For example, alkenes contain a C-C double bond, an electron-rich functional group with a nucleophilic  bond. Thus, alkenes react with electrophiles E+, but not with other electron rich species like nucleophiles (e.g. OH¯ or Br¯).

20. Nucleophile Attacking an Electrophile Alkyl halides possess an electrophilic carbon atom, so they react with electron-rich nucleophiles.

21. Intermolecular Forces & Organic Molecules

22. Ionic Interactions Exist in Ionic Compounds Ion-Ion Interaction • Ionic compounds contain oppositely charged particles held together by extremely strong electrostatic inter-actions. These ionic inter-actions are much stronger than the intermolecular forces present between covalent molecules.

23. Intermolecular Forces Between Molecules • Intermolecular forces are also referred to as noncovalent interactions or nonbonded interactions. • The nature of the forces between molecules depends on the functional group present. There are three different types of interactions, shown below in order of increasing strength: • van der Waals forces (London Dispersion) • dipole-dipole interactions • hydrogen bonding

24. Intermolecular Forces—van der Waals Forces or London Forces • They are weak interactions caused by momentary changes in electron density in a molecule. • They are the only attractive forces present in nonpolar compounds. Even though CH4 has no net dipole, at any one instant its electron density may not be completely symmetrical, resulting in a temporary dipole. This can induce a temporary dipole in another molecule. The weak interaction of these temporary dipoles constituents van der Waals forces.

25. van der Waals Forces-Surface Area • All compounds exhibit van der Waals forces. • The surface area of a molecule determines the strength of the van der Waals interactions between molecules. The larger the surface area, the larger the attractive force between two molecules, and the stronger the intermolecular forces.

26. van der Waals Forces-Polarizability • van der Waals forces are also affected by polarizability. • Polarizability is a measure of how the electron cloud around an atom responds to changes in its electronic environment. Larger atoms, like iodine, which have more loosely held valence electrons, are more polarizable than smaller atoms like fluorine, which have more tightly held electrons.

27. Intermolecular Forces—Dipole-Dipole Interactions • Dipole—dipole interactions are the attractive forces between the permanent dipoles of two polar molecules. • Consider acetone (below). The dipoles in adjacent molecules align so that the partial positive and partial negative charges are in close proximity. These attractive forces caused by permanent dipoles are much stronger than weak van der Waals forces.

28. Intermolecular Forces—Hydrogen Bonding • Hydrogen bonding typically occurs when a hydrogen atom bonded to O, N, or F, is electrostatically attracted to a lone pair of electrons on an O, N, or F atom in another molecule.

29. Summary of Intermolecular Forces Note: as the polarity of an organic molecule increases, so does the strength of its intermolecular forces.

30. Physical Properties of Organic Molecules

31. Quiz Review • Tuesday 5-6 pm • Wednesday 3-4 pm • Thursday 6-6:50am • You may come to one or all review sessions. • Bring questions. (No Questions-No Review) • If you can not make it to any of the review time then see me at my office hours.

32. Boiling Point • The boiling point of a compound is the temperature at which liquid molecules are converted into gas. • In boiling, energy is needed to overcome the attractive forces in the more ordered liquid state. • The stronger the intermolecular forces, the higher the boiling point. • For compounds with approximately the same molecular weight:

33. Comparing Boiling Points-molecules w/ different functional groups Consider the example below. Note that the relative strength of the intermolecular forces increases from pentane to butanal to 1-butanol. The boiling points of these compounds increase in the same order. • For two compounds with similar functional groups: • The larger the surface area, the higher the boiling point. • The more polarizable the atoms, the higher the boiling point.

34. Comparing Boiling Points-molecules w/ different functional groups Consider the examples below which illustrate the effect of size and polarizability on boiling points.

35. Separation of Molecules by Boiling Point Liquids having different boiling points can be separated in the laboratory using a distillation apparatus, shown in Figure 3.4.

36. Melting Point • The melting point is the temperature at which a solid is converted to its liquid phase. • In melting, energy is needed to overcome the attractive forces in the more ordered crystalline solid. • The stronger the intermolecular forces, the higher the melting point. • Given the same functional group, the more symmetrical the compound, the higher the melting point.

37. Melting Point • Because ionic compounds are held together by extremely strong interactions, they have very high melting points. • With covalent molecules, the melting point depends upon the identity of the functional group. For compounds of approximately the same molecular weight:

38. Melting Point Trends • The trend in melting points of pentane, butanal, and 1-butanol parallels the trend observed in their boiling points.

39. The Symmetry of a Molecule Determines its Melting Point • A compact symmetrical molecule like neopentane packs well into a crystalline lattice whereas isopentane, which has a CH3 group dangling from a four-carbon chain, does not. Thus, neopentane has a much higher melting point.

40. Solubility • Solubility is the extent to which a compound, called a solute, dissolves in a liquid, called a solvent. • In dissolving a compound, the energy needed to break up the interactions between the molecules or ions of the solute comes from new interactions between the solute and the solvent.

41. Solubility-comparing properties of solvents and solutes • Compounds dissolve in solvents having similar kinds of intermolecular forces. • “Like dissolves like.” • Polar compounds dissolve in polar solvents. Nonpolar or weakly polar compounds dissolve in nonpolar or weakly polar solvents. • Water is very polar solvent (forms hydrogen bonds with solutes). • Most ionic compounds are soluble in water, but insoluble in organic solvents. • Many organic solvents are either nonpolar, like carbon tetrachloride (CCl4) and hexane [CH3(CH2)4CH3], or weakly polar, like diethyl ether (CH3CH2OCH2CH3).

42. Solubility-dissolving ionic compounds • To dissolve an ionic compound, the strong ion-ion interactions must be replaced by many weaker ion-dipole interactions.

43. Solubility-the five carbon rule for organic compounds • An organic compound is water solubleONLY if it contains one polar functional group capable of hydrogen bonding with the solvent for every five C atoms it contains. • For example, compare the solubility of butane and acetone in H2O and CCl4.

44. Solubility-organic compounds which are soluble in water • Since butane and acetone are both organic compounds, they are soluble in the organic solvent CCl4. Butane, which is nonpolar, is insoluble in H2O. Acetone is soluble in H2O because it contains only three C atoms and its O atom can hydrogen bond with an H atom of H2O.

45. Organic Molecules which are not Water soluble • Cholesterol has 27 carbon atoms and only one OH group. Its carbon skeleton is too large for the OH group to solubilize by hydrogen bonding, so cholesterol is insoluble in water.

46. Solubility • The nonpolar part of a molecule that is not attracted to H2O is said to be hydrophobic. • The polar part of a molecule that can hydrogen bond to H2O is said to be hydrophilic. • In cholesterol, for example, the hydroxy group is hydrophilic, whereas the carbon skeleton is hydrophobic.

49. Application—Soap Soap: Soap molecules have two distinct parts—a hydrophilic portion composed of ions called the polar head, and a hydrophobic carbon chain of nonpolar C—C and C—H bonds, called the nonpolar tail.

50. The Cell Membrane