NUCLEOPHILES ELECTROPHILES

8/11/2019 NUCLEOPHILES ELECTROPHILES

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Functional Groups

Afunctional group is an atom or a group of atoms withcharacteristic chemical and physical properties.It is thereactive part of the molecule.

Most organic compounds have CC and CH bonds.However, many organic molecules possess otherstructural features:

Heteroatomsatoms other than carbon or hydrogen.

Bondsthe most common bonds occur in CCand CO double bonds.

These structural features distinguish one organicmolecule from another. They determine a molecule

sgeometry, physical properties, and reactivity, andcomprise what is called a functional group.

Introduction to Organic Molecules and Functional Groups


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Heteroatoms and bonds confer reactivity on a particularmolecule.

Heteroatoms have lone pairs and create electron-deficient sites on carbon.

Bonds are easily broken in chemical reactions. Abond makes a molecule a base and a nucleophile.

Don

t think that the CC and CH bonds are unimportant.They form the carbon backbone or skeleton to which the

functional group is attached.


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Ethane: This molecule has only CC and CH bonds, so it hasno functional group. Ethane has no polar bonds, no lone pairs,and no bonds, so it has no reactive sites. Consequently, ethaneand molecules like it are very unreactive.

Ethanol: This molecule has an OH group attached to itsbackbone. 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 verydifferent from the properties of ethane.


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


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OHH2SO4

OH H OSO3H

OH2+ HSO4

H2SO4H3C CH3 No Reaction

OHNaH

OH

Na H O + H2Na

NaHH3C CH3

No Reaction


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Aromatic hydrocarbons are so named because many of theearliest known aromatic compounds had strong characteristicodors.

The simplest aromatic hydrocarbon is benzene. The six-membered ring and three bonds of benzene comprise a singlefunctional group.

When a benzene ring is bonded to another group, it is called a

phenyl group.


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Compounds Containing the C=O Group:

This group is called a carbonyl group.

The polar CO bond makes the carbonyl carbon anelectrophile, while the lone pairs on O allow it to reactas a nucleophile and base.

The carbonyl group also contains a bond that is moreeasily broken than a CO bond.


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It should be noted that the importance of a functionalgroup cannot be overstated.

A functional group determines all of the followingproperties of a molecule:

Bonding and shape

Type and strength of intermolecular forces

Physical properties

Nomenclature

Chemical reactivity


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Which of the following is an ester?

H3CH2CH2C

O

CH3 H3CH2CH2C

O

NH2

H3CH2CH2C

O

OH

H3CH2CH2C

O

A B

CD

O

CH3

H3CH2CH2C

O

C

O

CH3


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Which of the following is an amide?

H3CH2CH2C

O

CH3 H3CH2CH2C

O

NH

H3

CH2

CH2

C

O

OHH2

N

A B

CD

O

CH3

CH2CH3

H3CH2CH2C

O

NH

B

CH2CH3H2N

C

O

CH3


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Intermolecular Forces

Intermolecular forces are interactions that exist betweenmolecules. Functional groups determine the type andstrength of these interactions.

There are several types of intermolecular interactions.

Ionic compounds containoppositely charged particlesheld together by extremelystrong electrostatic inter-

actions. These ionic inter-actions are much strongerthan the intermolecular forcespresent between covalent

molecules.


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Covalent compounds are composed of discretemolecules.

The nature of the forces between molecules depends onthe functional group present. There are three differenttypes of interactions, shown below in order ofincreasing strength:

van der Waals forces

dipole-dipole interactions

hydrogen bonding


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van der Waals Forces

van der Waals forces are also known as London forces.

They are weak interactions caused by momentary changes inelectron density in a molecule.

They are the only attractive forces present in nonpolarcompounds.

Even though CH4has nonet dipole, at any oneinstant its electron densitymay not be completelysymmetrical, resulting in atemporary dipole. This can

induce a temporary dipolein another molecule. Theweak interaction of thesetemporary dipolesconstitutes van der Waals

forces.


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All compounds exhibit van der Waals forces.

The surface area of a molecule determines the strength of thevan der Waals interactions between molecules. The larger thesurface area, the larger the attractive force between twomolecules, and the stronger the intermolecular forces.

Figure 3.1Surface area and

van der Waals forces


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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 looselyheld valence electrons,

are more polarizable thansmaller atoms likefluorine, which have moretightly held electrons.Thus, two F2molecules

have little attractive forcebetween them since theelectrons are tightly heldand temporary dipolesare difficult to induce.


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Dipole-Dipole Interactions

Dipoledipole interactions are the attractive forces betweenthe permanent dipoles of two polar molecules.

Consider acetone (below). The dipoles in adjacent moleculesalign so that the partial positive and partial negative chargesare in close proximity. These attractive forces caused by

permanent dipoles are much stronger than weak van der Waalsforces.


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Hydrogen bonding typically occurs when a hydrogenatom bonded to O, N, or F, is electrostatically attractedto a lone pair of electrons on an O, N, or F atom inanother molecule.

Hydrogen Bonding


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Note: as the polarity of an organic molecule increases, sodoes the strength of its intermolecular forces.


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What type of intermolecular forces are exhibited by eachmolecule?

N

O

OH

VDW VDW and DD

VDW, DDand HB

VDW


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Physical PropertiesBoiling Point

The boiling point of a compound is the temperature at whichliquid molecules are converted into gas.

In boiling, energy is needed to overcome the attractive forcesin the more ordered liquid state.

The stronger the intermolecular forces, the higher theboiling point.

For compounds with approximately the same molecularweight:


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Consider the example below. Note that the relativestrength of the intermolecular forces increases from

pentane to butanal to 1-butanol. The boiling points ofthese 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 boilingpoint.


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Consider the examples below which illustrate the effect ofsize and polarizability on boiling points.

Figure 3.2Effect of surface area and

polarizability on boiling point


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Which has the higher boiling point and why?

O

AB

A has only VDW, while B has both

VDW and DD interactions

O

B


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AB

O

OHH3CH2C

O

OCH3H3CH2C

A

O

OHH3CH2C

A had VDW, DDD and H-bonding, while B

lacks H-bonding


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H3C

(CH2)5

H3

C

H3C

(CH2)20

H3C

A B

H3C

(CH2)20

H3C

B

Both A and B only have VDW interactions, butB has the higher bp b/c of a larger surfacearea.


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The melting point is the temperature at which a solid isconverted 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.

Melting Point


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Because ionic compounds are held together byextremely strong interactions, they have very high

melting points. With covalent molecules, the melting point depends

upon the identity of the functional group. Forcompounds 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.


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Symmetry also plays a role in determining the melting points ofcompounds having the same functional group and similarmolecular weights, but very different shapes.

A compact symmetrical molecule like neopentane packs well intoa crystalline lattice whereas isopentane, which has a CH3groupdangling from a four-carbon chain, does not. Thus, neopentanehas a much higher melting point.


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Which has the higher melting point and why?

NH2

BA

B has stronger intermolecular forces (DD andHBZ).

NH2

B


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AB

A

Both only have VDW forces, so A has the highermp b/c it is more symmetrical. Closer packingmeans higher mp.


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

In dissolving acompound, theenergy needed to

break up theinteractionsbetween themolecules or ions

of the solute comesfrom newinteractionsbetween the soluteand the solvent.

Solubility


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Compounds dissolve in solvents having similar kinds ofintermolecular forces.

Like dissolves like. Polar compounds dissolve in polar solvents. Nonpolar or

weakly polar compounds dissolve in nonpolar or weaklypolar solvents.

Water and organic solvents are two different kinds ofsolvents. Water is very polar and is capable of hydrogenbonding with a solute. Many organic solvents are eithernonpolar, like carbon tetrachloride (CCl4) and hexane

[CH3(CH2)4CH3], or weakly polar, like diethyl ether(CH3CH2OCH2CH3).

Most ionic compounds are soluble in water, but insolublein organic solvents.


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An organic compound is water soluble only if it containsone polar functional group capable of hydrogen bonding

with the solvent for every five C atoms it contains. Forexample, compare the solubility of butane and acetone inH2O and CCl4.


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Since butane and acetone are both organic compounds

having a C

C and C

H backbone, they are soluble inthe organic solvent CCl4. Butane, which is nonpolar, isinsoluble in H2O. Acetone is soluble in H2O because itcontains only three C atoms and its O atom canhydrogen bond with an H atom of H2O.


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To dissolve an ionic compound, the strong ion-ioninteractions must be replaced by many weaker ion-dipole

interactions.

Figure 3.4Dissolving an ionic

compound in H2O


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The size of an organic molecule with a polar functional groupdetermines its water solubility. A low molecular weight alcohol

like ethanol is water soluble since it has a small carbonskeleton of five C atoms, compared to the size of its polarOH group. Cholesterol has 27 carbon atoms and only one OHgroup. Its carbon skeleton is too large for the OH group tosolubilize by hydrogen bonding, so cholesterol is insoluble in

water.


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The nonpolar part of a molecule that is not attracted to H2O issaid to be hydrophobic.

The polar part of a molecule that can hydrogen bond to H2O issaid to be hydrophilic.

In cholesterol, for example, the hydroxy group is hydrophilic,whereas the carbon skeleton is hydrophobic.


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Which of the following are water soluable?

O

N

A B

C

O atom, 5 orless Cs

soluable

No O, N or F, nonpolar,not soluable

Has N, but more

than 5 C

s, so notsoluable


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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 (orelectrophilic) sites and electron-rich (or nucleophilic)sites in a molecule. Molecules react at these sites.


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An electron-deficient carbon reacts with anucleophile, symbolized as :Nu.

An electron-rich carbon reacts with an electrophile,symbolized as E+.

For example, alkenes contain an electron rich doublebond, and so they react with electrophiles E+.


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


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Considering only electron density, will each reactionoccur?

Cl

+ OH

E+ Nu-Yes

+ Br-

Nu- Nu-

No

O

ClO

+ OCH3

Nu-

E+

Yes

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