1 functional groups a functional group is an atom or a group of atoms with characteristic chemical...
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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.
Introduction to Organic Molecules and Functional Groups
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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.
Don’t think that the C—C and C—H 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 C—C and C—H bonds, so it has no functional group. Ethane has no polar bonds, no lone pairs, and no bonds, so it has no reactive sites. Consequently, ethane and molecules like it are very 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.
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Hydrocarbons are compounds made up of only the elements carbon and hydrogen. They may be aliphatic or aromatic.
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OHH2SO4
OH H OSO3H
OH2+ HSO4
H2SO4H3C CH3No Reaction
OHNaH
OHNa H O + H2Na
NaHH3C CH3
No Reaction
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• Aromatic hydrocarbons are so named because many of the earliest known aromatic compounds had strong characteristic odors.
• The simplest aromatic hydrocarbon is benzene. The six-membered ring and three bonds of benzene comprise a single functional 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 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.
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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
Type and strength of intermolecular forces
Physical properties
Nomenclature
Chemical reactivity
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Which of the following is an aldehyde?
H3CH2CH2C
O
CH3H3CH2CH2C
O
H
H3CH2CH2C
O
OH
H3CH2CH2C
O
CH3
A B
CD
H3CH2CH2C
O
H
B
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Which of the following is an ester?
H3CH2CH2C
O
CH3H3CH2CH2C
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
CH3H3CH2CH2C
O
NH
H3CH2CH2C
O
OHH2N
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 between molecules. Functional groups determine the type and strength of these interactions.
• There are several types of intermolecular interactions.
• 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.
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• Covalent compounds are composed of discrete molecules.
• 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
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 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.
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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 between two molecules, 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 loosely held valence electrons, are more polarizable than smaller atoms like fluorine, which have more tightly held electrons. Thus, two F2 molecules have little attractive force between them since the electrons are tightly held and temporary dipoles are difficult to induce.
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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.
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• 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.
Hydrogen Bonding
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Note: as the polarity of an organic molecule increases, so does the strength of its intermolecular forces.
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What type of intermolecular forces are exhibited by each molecule?
N
O
OH
VDW VDW and DD
VDW, DD and HB
VDW
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Physical Properties—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:
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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.
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Consider the examples below which illustrate the effect of size 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
A B
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
H3CH3C
(CH2)20
H3C
A B
H3C
(CH2)20
H3C
B
Both A and B only have VDW interactions, but B has the higher bp b/c of a larger surface area.
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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.
Melting Point
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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.
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• Symmetry also plays a role in determining the melting points of compounds having the same functional group and similar molecular weights, but very different shapes.
• 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.
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Which has the higher melting point and why?
NH2
BA
B has stronger intermolecular forces (DD and HBZ).
NH2
B
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A B
A
Both only have VDW forces, so A has the higher mp b/c it is more symmetrical. Closer packing means higher mp.
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• 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.
Solubility
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• 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 and organic solvents are two different kinds of 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.
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• An organic compound is water soluble only 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.
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• Since butane and acetone are both organic compounds having a C—C and C—H backbone, 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.
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• To dissolve an ionic compound, the strong ion-ion interactions must be replaced by many weaker ion-dipole interactions.
Figure 3.4Dissolving an ioniccompound in H2O
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• The size of an organic molecule with a polar functional group determines its water solubility. A low molecular weight alcohol like ethanol is water soluble since it has a small carbon skeleton of five C atoms, compared to the size of its polar OH group. 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.
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• 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.
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Which of the following are water soluable?
O
N
A B
C
O atom, 5 or less Cs
soluable
No O, N or F, nonpolar, not soluable
Has N, but more than 5 C’s, so not soluable
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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 (or electrophilic) 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 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+.
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On the other hand, alkyl halides possess an electrophilic carbon atom, so they react with electron-rich nucleophiles.
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Considering only electron density, will each reaction occur?
Cl
+ OH
E+ Nu-
Yes
+ Br-
Nu- Nu-
No
O
ClO
+ OCH3
Nu-
E+
Yes