CM1501-Week 2B-Chapter 3-Intro Org Reaction

CM1501 Week 2B-Chapter 3-Intro Org Mol

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 and additionally other structural features:  Heteroatoms—atoms other than carbon or hydrogen.   Bonds—the most common  bonds occur in C=C and C=O double bonds.

The above 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. 1

CM1501 Week 2B-Chapter 3-Intro Org Mol

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.

C—C and C—H single bonds form the carbon backbone or skeleton to which the functional group is attached.

2

CM1501 Week 2B-Chapter 3-Intro Org Mol

Functional Groups • Ethane contains only C-C and C-H bonds, no polar bonds, no lone pairs, and no  bonds. This makes it very unreactive.

• Ethanol has an OH group (hydroxyl group). Its lone pairs and polar bonds make it reactive with a variety of reagents. The hydroxyl group makes the properties of ethanol very different from the properties of ethane.

3

CM1501 Week 2B-Chapter 3-Intro Org Mol

Functional Groups Classifications 1.

Hydrocarbons

2.

Compounds containing a C-Z sigma bond (Z = an electronegative element)

3.

Compounds containing a C=O group

4

CM1501 Week 2B-Chapter 3-Intro Org Mol

Functional Groups Hydrocarbons Compounds made up of only carbon and hydrogen

5

CM1501 Week 2B-Chapter 3-Intro Org Mol

Functional Groups Hydrocarbons • Aromatic hydrocarbons are so named because many of the earliest known aromatic compounds had strong characteristic odors.

sp2

6

CM1501 Week 2B-Chapter 3-Intro Org Mol

Functional Groups Molecules Containing C-Z  Bonds

Alkyl halide Alcohol

Ether

Amine Thiol Sulfide 7

CM1501 Week 2B-Chapter 3-Intro Org Mol

Functional Groups Compounds Containing the C=O Group • C=O is called “carbonyl group”. • The polar C—O bond makes the carbonyl carbon an electrophile, while the lone pairs on O allow it to react as a nucleophile and base. • The carbonyl group also contains a  bond that is more easily broken than a C—O  bond.

8

CM1501 Week 2B-Chapter 3-Intro Org Mol

Functional Groups Compounds Containing the C=O Group

Aldehyde

Ketone Carboxylic acid Ester

Amide Acid chloride

9

CM1501 Week 2B-Chapter 3-Intro Org Mol

Functional Groups Compounds Containing Both C-Z and C=O Groups

10

CM1501 Week 2B-Chapter 3-Intro Org Mol

Functional Groups Overview

A functional group and connectivities among them determine all of the following properties of a molecule: 

Bonding and shape



Type and strength of intermolecular forces



Physical properties



Chemical reactivity



Nomenclature

11

CM1501 Week 2B-Chapter 3-Intro Org Mol

Intermolecular Forces

Some textbooks may classify id-id and pdd as vdW. However, for the purpose of this module, we will consider vdW as reference to id-id only.

• They exist between molecules. • Their type and strength are determined by functional groups • As the polarity of an organic molecule increases, so does the strength of its intermolecular forces.

Van der Waals (Dispersion Forces)

Dipole-dipole Hydrogen bonding Ionic bond 12

CM1501 Week 2B-Chapter 3-Intro Org Mol

Intermolecular Forces Ionic Bonds

Ionic

compounds

contain

oppositely charged particles held together

by

electrostatic ionic

extremely

strong

interactions.

These

inter-actions

are

much

stronger than the intermolecular forces present between covalent molecules.

13

CM1501 Week 2B-Chapter 3-Intro Org Mol

Intermolecular Forces Van der Waals Forces • Van der Waals forces are also known as London dispersion 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 constitutes van der Waals forces. 14

CM1501 Week 2B-Chapter 3-Intro Org Mol

Intermolecular Forces Van der Waals Forces • 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, and the stronger the intermolecular forces.

Surface area and van der Waals forces

15

CM1501 Week 2B-Chapter 3-Intro Org Mol

Intermolecular Forces Van der Waals Forces Van der Waals forces are also affected by polarizability, a measure of how the electron cloud around an atom responds to changes in its electronic environment.

More tightly held electrons Difficult to induce temporary dipoles

Polarizability depends on the size of anion

More loosely held electrons Easier to induce temporary dipoles

16

CM1501 Week 2B-Chapter 3-Intro Org Mol

Intermolecular Forces Dipole-Dipole Interactions • Dipole—dipole interactions are the attractive forces between the permanent dipoles of two polar molecules. • Stronger than weak van der Waals forces.

17

CM1501 Week 2B-Chapter 3-Intro Org Mol

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

18

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Boiling Point (bp)

More symmetrical means more spherical.

• The boiling point 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. • Given the same functional group, the more symmetrical the compound, the lower the boiling point. • For compounds with approximately the same molecular weight:

19

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Boiling Point (bp) Due to the fact that the relative strength of the intermolecular forces increases from pentane to butanal to 1-butanol, their boiling points increase in the same order.

Van der Waals

Dipole-dipole

Hydrogen bonding

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. 20

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Boiling Point (bp)

Effect of surface area and polarizability on boiling point

21

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Melting Point (mp)

• 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.

22

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Melting Point (mp) • 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:

23

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Melting Point (mp)

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

Van der Waals

Dipole-dipole

Hydrogen bonding

24

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Melting Point (mp) • 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.

bp = 28 oC

bp = 10 oC

Symmetry Effect

Lower boiling point because id-id interactions is weaker due to smaller surface area.

25

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Melting Point (mp)

bp = 28 oC

bp = 10 oC

Symmetry Effect

26

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Solubility • Solubility is the extent to which a compound, called a solute, dissolves in a liquid, called a solvent.

27

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Solubility • To dissolve an ionic compound such as Na+Cl-, the strong ion-ion interactions must be replaced by many weaker ion-dipole interactions.

Many

weak

ion–

dipole interactions work

together

to

H

compensate for the stronger

O

H

ionic

bonds.

Dissolving an ionic compound in H2O

28

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Solubility

• Polar compounds dissolve in polar solvents. Nonpolar or weakly polar compounds dissolve in nonpolar or weakly polar solvents.

• Water is very polar and is capable of hydrogen bonding with a solute. Many organic solvents are either nonpolar, like carbon tetrachloride (CCl4) and hexane [CH3(CH2)4CH3], or weakly polar, like diethyl ether (CH3CH2OCH2CH3). • Most ionic compounds are soluble in water, but insoluble in organic solvents.

Like dissolves like 29

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Solubility

• An organic compound is water soluble only if it contains polar functional groups capable of hydrogen bonding with the solvent for every five C atoms it contains.

Like dissolves like 30

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Solubility • The size & polar functional group determine the water solubility. A low molecular weight alcohol like ethanol is water soluble as it has a carbon skeleton of  five C atoms, compared to the size of its polar OH group. • The nonpolar part of a molecule that is not attracted to H2O is said to be hydrophobic. • The polar part of a molecule is said to be hydrophilic. • Cholesterol: 27 carbon atoms + one OH group; insoluble in water.

31

CM1501 Week 2B-Chapter 3-Intro Org Mol

Physical Properties Solubility

32

CM1501 Week 2B-Chapter 3-Intro Org Mol

Chemical Properties • 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 electronrich (or nucleophilic) sites in a molecule.

Influence of Functional Groups on Reactivity 33

CM1501 Week 2B-Chapter 3-Intro Org Mol

Chemical Properties

base

Influence of Functional Groups on Reactivity 34

CM1501 Week 2B-Chapter 3-Intro Org Mol

Chemical Properties • An electron-deficient carbon reacts with a nucleophile, symbolized as :Nu¯. • An electron-rich carbon reacts with an electrophile, symbolized as E+. For example, alkenes contain an electron rich double bond, and so they react with electrophiles E+.

Influence of Functional Groups on Reactivity 35

CM1501 Week 2B-Chapter 3-Intro Org Mol

Chemical Properties

On the other hand, alkyl halides possess an electrophilic carbon atom, so they react with electron-rich nucleophiles.

Influence of Functional Groups on Reactivity 36