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

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CM1501 Week 2B-Chapter 3-Intro Org Mol

• 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.

Functional Groups

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.

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• 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.

Functional Groups

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• 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.

Functional Groups

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Functional Groups

1. Hydrocarbons

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

3. Compounds containing a C=O group

Classifications

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Compounds made up of only carbon and hydrogen

Functional GroupsHydrocarbons

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

Functional GroupsHydrocarbons

sp2

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Functional Groups

Alkyl halide

Alcohol

Ether

Amine

Thiol

Sulfide

Molecules Containing C-Z Bonds

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• 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.

Functional GroupsCompounds Containing the C=O Group

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Functional Groups

Aldehyde

Ketone

Carboxylic acid

Ester

Amide

Acid chloride

Compounds Containing the C=O Group

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

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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

Functional GroupsOverview

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• 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.

Intermolecular Forces

Van der Waals

Hydrogen bonding

Ionic bond

(Dispersion Forces)

Dipole-dipole

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.

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Intermolecular Forces

Ionic compounds contain oppositely charged particles held together by extremely strong electrostatic interactions. These ionic inter-actions are much stronger than the intermolecular forces present between covalent molecules.

Ionic Bonds

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• 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.

Intermolecular ForcesVan der Waals Forces

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• 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


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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

More loosely held electrons

Easier to induce temporary dipoles

Polarizability depends on the size of anion

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• Dipole—dipole interactions are the attractive forces between the permanent dipoles of two polar molecules.

• Stronger than weak van der Waals forces.

Intermolecular ForcesDipole-Dipole Interactions

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• 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.

Intermolecular ForcesHydrogen Bonding

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• 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:

Physical PropertiesBoiling Point (bp)

More symmetrical means more spherical.

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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.

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.


Van der Waals Hydrogen bondingDipole-dipole

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Effect of surface area and polarizability on boiling point


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• 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.

Physical PropertiesMelting Point (mp)

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• 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:


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


Van der Waals Hydrogen bondingDipole-dipole

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• A compact symmetrical molecule like neopentane packs well into acrystalline lattice whereas isopentane, which has a CH3 group dangling from a four-carbon chain, does not. Thus, neopentane has a much higher melting point.


Symmetry Effect

bp = 28 oC bp = 10 oC

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

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Physical Properties

Symmetry Effect

Melting Point (mp)

bp = 28 oC bp = 10 oC

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

Physical PropertiesSolubility

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

Dissolving an ionic compound in H2O


Many weak ion–

dipole interactions

work together to

compensate for the

stronger ionic

bonds.

O H

H

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• 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

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• 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

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• 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.

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• 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.

Chemical Properties

Influence of Functional Groups on Reactivity

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Chemical Properties

base

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• 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+.


Chemical Properties

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


Chemical Properties

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