Transcript Alcohols - Structure - University of Nebraska Omaha

Alcohols, Ethers and Thiols
Structure of Alcohols
• The functional group of an alcohol is an OH (hydroxyl) group bonded to an sp3
hybridized carbon.
• Bond angles about the hydroxyl oxygen atom
are approximately 109.5°.
• Oxygen is also sp3 hybridized.
• Two sp3 hybrid orbitals form sigma bonds to
carbon and hydrogen.
• The remaining two sp3 hybrid orbitals each
contain an unshared pair of electrons.
Nomenclature of Alcohols
• IUPAC names
• The parent chain is the longest chain that contains the -OH group.
• Number the parent chain in the direction that gives the -OH group the lower
number.
• Change the suffix -e to -ol.
• Common names
• Name the alkyl group bonded to oxygen followed by the word alcohol.
• Examples:
• Compounds containing
• two -OH groups are named as diols,
• three -OH groups are named as triols.
• -OH groups on adjacent carbons are called glycols.
• Ethylene glycol (toxic) and propylene glycol (nontoxic)
are used in antifreeze.
• Glycerin is used in moisturizers and play bubbles.
• Unsaturated alcohols
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The double bond is shown by the infix -en-.
The hydroxyl group is shown by the suffix -ol.
Number the chain to give OH the lower number.
Alcohol has higher priority than alkene.
Hex-3-en-1-ol
Physical Properties
• Alcohols are polar solvents.
• Polarity of an alcohol originates in the C-O-H bond in methanol.
a) Partial positive charges on carbon and hydrogen and a partial negative
charge on oxygen.
b) An electron density map showing the partial negative charge in red and the
partial positive charge in blue.
Hydrogen Bonding in Alcohols
• Alcohols condense to the liquid state because of hydrogen bonding.
• The strength of hydrogen bonding in alcohols is approximately 2 to 5
kcal/mol.
• Hydrogen bonds are considerably weaker than covalent bonds (for example,
110 kcal/mol for an O–H bond).
• Nonetheless, hydrogen bonding can have a significant effect on physical
properties.
• Alcohols have relatively high boiling points and melting points
because of hydrogen bonding.
The association of ethanol molecules in the liquid state.
• Note the huge difference in boiling points and solubility between
analogous alkanes and alcohols.
• Boiling points increase with chain size.
• Water solubility of alcohols decreases with increasing chain size.
(molecules get more hydrophobic)
Acidity of Alcohols
• Most alcohols are about the same or slightly weaker acids than water.
Basicity of Alcohols
• In the presence of strong acids, the oxygen atom of an alcohol
behaves as a weak base.
• Proton transfer from the strong acid forms an oxonium ion.
• Thus, alcohols can function as both weak acids and weak bases.
Overview of Reactions of Alcohols
• Reactions with active metals
• Product is the conjugate base of the alcohol.
• Substitution of halogen for hydroxyl group.
• Reaction can be with HX or SOCl2.
• Dehydration
• Acid-catalyzed elimination to form alkene.
• Oxidation
• Use chromic acid, H2CrO4, (CrO3/H2SO4) to convert 1 alcohol to aldehyde or
carboxylic acid
• Use chromic acid to convert 2 alcohol to ketone
Reaction with Active Metals
• Alcohols react with Li, Na, K, and other active metals to liberate
hydrogen gas and form metal alkoxides.
• Na is oxidized to Na+ and H+ is reduced to H2.
• Alkoxides are somewhat stronger bases that OH–.
• Alkoxides can be used as bases in -elimination reactions.
• They can be used also as nucleophiles in nucleophilic substitution reactions.
Conversion of ROH to RX with HX
• Water-soluble 3° alcohols react very rapidly with moderate
concentrations of HCl, HBr, and HI.
• Low-molecular-weight 1° and 2° alcohols are unreactive under
these conditions.
• Water-insoluble 3° alcohols react by bubbling gaseous HCl
through a solution of the alcohol dissolved in diethyl ether or THF.
• 1° and 2° alcohols require concentrated HBr and HI to form alkyl
bromides and iodides.
• 3° Alcohols react with HX by an SN1 mechanism
• Step 1: Add a proton. Rapid and reversible proton transfer from the acid to
the —OH group.
• This proton transfer converts the OH– from a poor leaving group, to H2O, a
much better leaving group.
• Step 2: Break a bond to form a stable molecule or ion. Loss of H2O gives a
3° carbocation.
• Step 3: Reaction of an electrophile and a nucleophile to form a new
covalent bond completes the reaction.
• 1° alcohols react with HX by an SN2 mechanism.
• Step 1: Add a proton. Proton transfer to OH converts OH–, a poor leaving
group, to H2O a better leaving group.
• Step 2: Reaction of a nucleophile and an electrophile to form a new
covalent bond and break a bond.
• Substitutions of –OH for –X are governed by a combination of
electronic and steric effects
Conversion of ROH to RX with SOCl2
• Thionyl chloride, SOCl2, is the most widely used reagent for
conversion of primary and secondary alcohols to alkyl chlorides.
Acid-Catalyzed Dehydration of Alcohols
• An alcohol can be converted to an alkene by elimination of H and OH
from adjacent carbons (a -elimination).
• 1° alcohols must be heated at high temperature in the presence of an acid
catalyst, such as H2SO4 or H3PO4.
• 2° alcohols undergo dehydration at somewhat lower temperatures.
• 3° alcohols often require temperatures at or only slightly above room
temperature.
• When isomeric alkenes are obtained, the more stable alkene (the one
with the greater number of substituents on the double bond) generally
predominates (Zaitsev’s rule).
Dehydration of a 2° Alcohol
• A three-step mechanism
• Step 1: Add a proton. Proton transfer from H3O+ to the –OH group converts
OH–, a poor leaving group, into H2O, a much better leaving group.
• Step 2: Break a bond to form a stable molecule or ion.
Loss of H2O gives a carbocation intermediate.
• Step 3: Take a proton away. Proton transfer from an adjacent carbon to
H2O gives the alkene and regenerates the acid catalyst.
Dehydration of a 1° Alcohol
• A two-step mechanism
• Step 1: Add a proton. Proton transfer from the acid gives
an oxonium ion.
• Step 2: Take a proton away and loss of H2O gives the
alkene and regenerates the acid catalyst.
Hydration-Dehydration Equilibrium
• Acid-catalyzed hydration of an alkene and dehydration of an alcohol
are competing processes.
• Large amounts of water favor alcohol formation.
• Scarcity of water or experimental conditions where water is removed (like high
temperatures to evaporate water) favor alkene formation.
Oxidation of Alcohols
• Oxidation of a 1° alcohol gives an aldehyde or a carboxylic acid,
depending on the oxidizing agent and experimental conditions.
• The most common oxidizing agent is chromic acid.
• Chromic acid oxidation of 1-octanol gives octanoic acid.
• To oxidize a 1° alcohol to an aldehyde, use a combination of chromium(VI)
oxide, hydrochloric acid and pyridine to form pyridinium chlorochromate, (PCC).
• PCC oxidation of geraniol gives geranial.
• Oxidation of a 2° alcohol gives a ketone only.
• Tertiary alcohols are not oxidized by either of these reagents; they are
resistant to oxidation.
Structure of Ethers
• The functional group of an ether is an oxygen atom bonded to two
carbon atoms.
• Oxygen is sp3 hybridized with bond angles of approximately 109.5°.
• In dimethyl ether, the C–O–C bond angle is 110.3°.
Nomenclature of Ethers
• IUPAC
• The longest carbon chain is the parent alkane.
• Name the -OR group as an alkoxy substituent.
• Hydroxyl group has priority over alkoxy group.
• Common names:
• Name the two alkyl branches bonded to oxygen followed by the word ether.
• Although cyclic ethers have IUPAC names, their common names
are more widely used.
Physical Properties of Ethers
• Ethers are polar molecules.
• Each C-O bond is polar covalent.
• However, only weak polar forces exist between ether molecules in the pure
liquid.
• Ethers are commonly used as polar aprotic solvents.
• Ethers are also used as anesthetics.
• Ethers have much lower boiling points than their isomeric
cousins, alcohols.
• Ethers are less soluble in water than alcohols.
Reactions of Ethers
• Ethers resemble hydrocarbons in their resistance to chemical
reaction.
• They do not react with strong oxidizing agents such as chromic acid, H2CrO4.
• They are not affected by most acids and bases at moderate temperatures.
• Because of their good solvent properties and general inertness to
chemical reaction, ethers are excellent solvents in which to perform
organic reactions.
Epoxides
• Epoxide: A cyclic ether in which oxygen is one atom of a threemembered ring.
• Ethylene oxide is synthesized from ethylene and O2.
• Other epoxides can be synthesized from an alkene by oxidation with a
peroxycarboxylic acid, RCO3H.
O
CH3COOOH
2,3-epoxybutane
cis-2-butene
CH3COOOH
cyclohexene
O
1,2-epoxycyclohexane
Acid-catalyzed Ring Opening of an Epoxide
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Epoxides react readily because of the angle strain in the three-membered ring.
Acid catalyst protonates epoxide oxygen atom.
Water (acting as a nucleophile) attacks least hindered carbon.
Reaction of an epoxide with aqueous acid gives a glycol.
The addition of the water is anti.
Nucleophiles other than water (NH3, OH-, OR-, SH-) can be used.
Such reactivity is in contrast to the lack of reactivity generally in ethers.
Structure of Thiols
• The functional group of a thiol is an -SH (sulfhydryl) group bonded to
an sp3 hybridized carbon.
Nomenclature of Thiols
• IUPAC names
• The parent chain is the longest chain containing the -SH group.
• Add -thiol to the name of the parent chain.
• Sulfhydryl group has lower priority than hydroxyl.
• Alternatively, indicate a lower priority -SH by the prefix mercapto.
• Common names
• Name the alkyl group bonded to sulfur followed by the word mercaptan.
Physical Properties of Thiols
• Low-molecular-weight thiols have a STENCH!
• Odorants are added to natural gas at a concentration of about 1 ppm.
• Thiols are less polar than alcohols.
• The difference in electronegativity between S and H is 2.5 – 2.1 = 0.4.
• Intermolecular forces of thiols are much different than that of alcohols
because of their low polarity.
• Thiols show little association by hydrogen bonding.
• Thiols have lower boiling points and are less soluble in water than alcohols of
comparable MW.
Chemical Properties of Thiols
• Thiols are stronger acids than alcohols.
• Thiols react with strong bases to form salts.
• Conjugate bases of thiols make good nucleophiles.
Reactions of Thiols
• Thiols are oxidized by a variety of oxidizing agents, including O2, to
disulfides.
• Disulfides, in turn, are easily reduced to thiols by several reagents.
• This easy interconversion between thiols and disulfides is very important in
protein chemistry (the amino acid, cysteine, is a thiol).