Chemistry Genius - Properties and Reactions of Alcohols and Ethers lyrics

Alcohols Nomenclature For alkane alcohols, the -e is dropped and an -oh is added. In more complicated branched systems, the name is based off of the longest chain containing the hydroxy group, not necessarily the longest chain in the molecule. To locate where in the chain the the branches are, the carbons of the longest chain is numbered starting from the carbon closest to the OH. The other substituents are added as prefixes. When there is more than one hydroxy group, the suffixes -diol and -triol, etc are used. Cyclic alcohols are called cycloalkanols. The carbon carrying the hydroxy group is automatically carbon number 1. Like haloalkanes, alcohols can be primary, secondary, or tertiary, depending on the R groups attatched to the central carbon. Physical Properties Oxygen is highly electronegative. The resultant asymmetry of charge distribution polarizes the O-H bond and creates a dipole. This polarity and the presence of hydrogen results in hydrogen bonding, which raises the boiling point much higher than the comparative alkanes and haloalkanes. Furthermore, the polar OH group is hydrophilic and soluble in water, while the alkyl chains are hydrophobic and soluble in nonpolar solvents. The larger the alkyl portion of the alcohol, the lower its aqueous solubility. However, the lower alcohols are common polar, protic solvents for SN2 reactions. Acidic and Basic Alcohols Alcohols are weakly acidic. The deprotonation of the hydroxy group in alcohols gives alkoxide ions. This reaction can be carried out by a base stronger than alkoxide or by an alkali metal via reduction. Steric disruption of solvation and hydrogen bonding to the alkoxide serve to weaken the acid. Thus, tertiary alcohols make the weakest acids, with methanol the strongest. Halogens increase acidity via the inductive effect. The two lone pairs on oxygen makes alcohols basic, and protonation creates alkyloxonium ions, which is a good leaving group. Primary alcohols require strong nucleophiles for deprotonation and only undergo SN2. However, an increase in carbocation stability for secondary and tertiary alcohols means that they can undergo SN1 and E1 reactions. Synthesis Nucleophilic Substitution In a laboratory, alcohols can be prepared via both bimolecular and unimolecular nucleophilic substitution, with hydroxide and water as the respective nucleophiles. However, these reactions suffer from severe drawbacks. Hindered alcohols can undergo E2 reactions instead, while tertiary carbocations may undergo E1 reactions. Use of polar aprotic, solvents may counteract these issues. To specifically prevent an E2 reaction in sterically hindered cases, a less basic solvent than water can be used--acetate. The akyl acetate would then be converted via an aqueous hydroxide. Oxidation-Reduction An aldehyde can be reduced to a primary alcohol, and a ketone to a secondary alcohol. In essence, two hydrogen atoms are delivered across the carbonyl group. In practicality, a hydride and a proton are either simultaneously or sequentially delivered across the double bond. The preferred reagents are sodium borohydride and lithium aluminum hydride. Reduction can be reversed by chromium reagents. However, aqueous Cr(VI) overoxidize primary alcohols into carboxylic acids. This does not occur in the absence of water. Therefore, a water free form of Cr(VI) is used: pyridinium chlorochromate. First the chromic acid forms a chromic ester with the alcohol. Next, water or pyridine removes a hydrogen next to the alcoholic oxygen and Cr(IV) leaves. Organometallic Reagents Organometallic reagents are sources of nucleophilic carbon. Organometallic compounds are prepared by reacting a haloalkene with a metal in diethyl ether or oxacyclopentane solution. The highly polar metal-carbon bond can be symbolized by placing the full negative charge on the carbon, forming a carbanion. Because carbon is not particularly electronegative, the alkyl group is extremely basic. These alkylmetals can be used to turn alcohols into alkoxides, hydrolyze water, converting haloalkanes into alkanes, and synthesize alcohols. Carbocation Rearrangements Carbocations can undergo hydride or alkyl shift and in all likelihood react further via SN1 or E1 pathways. Alkyl shifts can result in ring expansion as the positive charge is formally exchanged with the alkyl group forming on end of the ring. Esters from Alcohols and Haloalkane Synthesis Organic esters are formed by reacting alcohol with carboxylic acids. Inorganic esters are inorganic acids with an alkyl group in lieu of carboxylic hydrogen. Inorganic esters can be synthesized from an alcohol, then used to create a haloalkane. Phosphorus tribromide is used to synthesize bromoalkane from primary or secondary alcohols. Thionyl chloride is used to convert alcohols into chloroalkanes. Alkyl sulfonates work by replacing the hydroxy group for itself, then in turn being displaced by a nucleophile. Haloalkanes can be synthesized from both primary and secondary/tertiary alcohol. Ethers Nomenclature Ethers are named like alkanes with an alkoxy substituent--alkoxyalkanes. The largest constituent defines the alkane. Cyclic ethers may have a heteroatom of oxygen, and are thus called heterocycles. Physical Properties Ethers are fairly unreactive and frequently used as solvents. The lack of hydrogen bonding results in a much lower boiling point than corresponding alcohols. As the size of the hydrocarbon increases, ethers become progressive less water soluble. The oxygen in ethers render it basic. Crown ethers can act as cation binders and solvate salts, and thus allow oxidations to occur in an organic solvent. Crown ethers and cryptands also function as ionophores. Synthesis Williamson Ether Synthesis The simplest way to synthesize an ether is to react an alkoxide with a primary haloalkane under SN2 conditions. However, the alkyl needs to be primary and unhindered to prevent E2 reactions. Williamson synthesis can be used to prepare cyclic ethers of various sizes, including oxacyclopropane. The rate of formation is deteremined by both entropy and enthalpy. Because this synthesis is based off of an SN2 mechanism, it is stereospecific and results in inversion of configuration. Alcohols and Mineral Acids Treating primary alcohols with a nucleophilic acid produces alkyloxonium ions. However, when nonnucleophilic acids are added at high temperatures, symmetric ethers are produced. One hydroxyl group is protonated by a strong acid to generate a water leaving group. Water is then displaced by another alcohol, which creates an alkoxyalkane. Symmetric ethers are produced when nonnucleophilic acids are added at high temperatures to secondary alcohols. In contrast, tertiary alcohols form mixed ethers. Reactions Ethers are typically chemically inactive. However, they can react with oxygen by radical mechanisms to create explosive peroxides. Ether can also be cleaved by strong acid. The acid protonates the oxygen in ether to form an alkyloxonium ions. In primary ethers and with a nucleophilic acid, SN2 reactions take place. In secondary ethers, either SN2 or SN1 reactions can occur. Ethers with tertiary alkyl groups can transform into tertiary carbocations and function as protecting groups to the hydroxy group. Oxalocyclopropanes have possible nucleophilic ring-opening reactions due to its ring strain. Acids catalyze ring openings. The nucleophilic anion attacks a carbon, causing the ether oxygen to leave. In asymmetric systems, the bond breaks at the less substituted side due to regioselectivity. When the ring opens at the sterocenter, inversion of configuration occurs. Sulfur an*logs of Alcohols and Ethers Sulfur is in the same group as oxygen, and thus behaves fairly similarly to oxygen. The sulfur an*logues to alcohol, R-SH, are called thiols. The sulfur an*logues to hydroxide, SH, are called mercaptos. The sulfur an*logues to ethers are called sulfides or thioethers. The RS group is called alkylthio; the anion called alkanethiolate. Thiols have weaker bonds to hydrogen, and therefore are more acidic. Because sulfur has d orbitals, it can hold up to 10 or 12 valence electrons. Disulfides can be formed. Sulfides can be oxidized into sulfones via a sulfoxide intermediate.