Strong bases common

Sodium hydroxide, NaOH, is a very strong base, commonly used as a drain deaner. Why is this chemical only helpful in unclogging natural materials, such as hair or grease, and not synthetic materials, such as plastic, or inorganic materials, such as sand ... 

The ylide is prepared by deprotonating a triphenylalkylphosphonium salt with a strong base, commonly an organometallic base such as butyllithium or phenyllithium. The hydrogens on the carbon that is bonded to the phosphorus of the salt are somewhat acidic because the carbanion of the conjugate base (the ylide) is stabilized by the inductive effect of the positive phosphorus atom. In addition, a resonance structure with five bonds to phosphorus makes a minor contribution to the structure and provides some additional stabilization. The triphenylalkylphosphonium salt can be prepared by an SN2 reaction of triphenylphosphine with the appropriate alkyl halide (see Section 10.9). 

Use of a Bulky Base If the substrate is prone to substitution, a bulky base can minimize the amount of substitution. Large alkyl groups on a bulky base hinder its approach to attack a carbon atom (substitution), yet it can easily abstract a proton (elimination). Some of the bulky strong bases commonly used for elimination are ferf-butoxide ion, diisopropylamine, triethylamine, and 2,6-dimethylpyridine. 

Just like some short straight carbon chains, some short branched carbon chains are given names and organic element symbols, The most common is the isopropyl group. Lithium diisopropylamide (also called LDA) is a strong base commonly used in organic synthesis. 

Other strong bases commonly used to form acetylide anions are sodium hydride and lithium diisopropylamide (LDA). 

Pr2NLi or (Me2CH)2NLi, is called Uthium diisopropylamide it is a strong base, commonly used in synthesis... 

LDA (Section 22.5) Lithium diisopropylamide, LiN( -C3H7)2, a strong base commonly used to convert carbonyl compounds into their enolate ions. 

Diisopropylamide ion [(CH ljCHljN is a strong base commonly used in organic reactions. Is it expected to be a stronger or weaker base than the amide ion ... 

Esters are alkylated in the presence of strong bases in aprotic solvents. A common combination is LDA in tetrabydrofuran at low temperatures. Equimolar amounts of base are sufficient and only the mono-carbanion Js formed. After addition of one mole of alkyl halide the products form rapidly, and no dialkylation, which is a problem in the presence of excess base, is possible. Addition of one more mole of LDA and of another alkyl halide leads to asymmetric dialkylation of one or-carbon atom in high yield (R.J. Cregge, 1973). 

The benzyl group has been widely used for the protection of hydroxyl functions in carbohydrate and nucleotide chemistry (C.M. McCloskey, 1957 C.B. Reese, 1965 B.E. Griffin, 1966). A common benzylation procedure involves heating with neat benzyl chloride and strong bases. A milder procedure is the reaction in DMF solution at room temperatiue with the aid of silver oxide (E. Reinefeld, 1971). Benzyl ethers are not affected by hydroxides and are stable towards oxidants (e.g. periodate, lead tetraacetate), LiAIH, amd weak acids. They are, however, readily cleaved in neutral solution at room temperature by palladium-catalyzed bydrogenolysis (S. Tejima, 1963) or by sodium in liquid ammonia or alcohols (E.J. Rcist, 1964). 

Strong and Weak Bases Just as the acidity of an aqueous solution is a measure of the concentration of the hydronium ion, H3O+, the basicity of an aqueous solution is a measure of the concentration of the hydroxide ion, OH . The most common example of a strong base is an alkali metal hydroxide, such as sodium hydroxide, which completely dissociates to produce the hydroxide ion. 

The most common strong base for titrating acidic analytes in aqueous solutions is NaOH. Sodium hydroxide is available both as a solid and as an approximately 50% w/v solution. Solutions of NaOH may be standardized against any of the primary weak acid standards listed in Table 9.7. The standardization of NaOH, however, is complicated by potential contamination from the following reaction between CO2 and OH . 

Titrimetric methods have been developed using acid-base, complexation, redox, and precipitation reactions. Acid-base titrations use a strong acid or strong base as a titrant. The most common titrant for a complexation titration is EDTA. Because of their... 

Ion-exchange resins are categorized by the nature of functional groups attached to a polymeric matrix, by the chemistry of the particular polymer in the matrix, and by the porosity of the polymeric matrix. There are four primary types of functionaHty strong acid, weak acid, strong base, and weak base. Another type consists of less common stmctures in specialty resins such as those which have chelating characteristics. 

Weak Base. Weak base anion-exchange resins may have primary, secondary, or tertiary amines as the functional group. The tertiary amine -N(CH2)2 is most common. Weak base resins are frequentiy preferred over strong base resins for removal of strong acids in order to take advantage of the greater ease in regeneration. 

Strong Base Anion Exchangers. As ia the synthesis of weak base anion exchangers, strong base resias are manufactured from styrenic as well as acryhc copolymers. Those based on copolymers of styrene and divinylben2ene are chloromethylated and then aminated. These reactions are the same as for the styrenic weakbase resias. The esseatial differeace is the amine used for amination. Trimethyl amine [75-50-3] N(CH2)3, and /V, /V- dim ethyl eth a n ol amine [108-01 -0] (CH2)2NCH2CH20H, are most commonly used. Both form quaternary ammonium functional groups similar to (8). 

If there is no phenyl substituent in the 3-position the amination ability decreases. The acyloxaziridine (104) yields only 11% of a semicarbazide derivative with piperidine. In the presence of strong bases an intramolecular amination competes. Compound (104) reacts with methoxide within a couple of seconds to give phenylhydrazine carboxylic ester (106), and with cyclohexylamine to give the substituted semicarbazide (107). A diaziridinone (105) is assumed to be the common intermediate, formed by an intramolecular reaction from deprotonated (104) (67CB2600). 

Dissociation extraction is the process of using chemical reac tion to force a solute to transfer from one liquid phase to another. One example is the use of a sodium hydroxide solution to extract phenolics, acids, or mercaptans from a hydrocarbon stream. The opposite transfer can be forced by adding an acid to a sodium phenate stream to spring the phenolic back to a free phenol that can be extrac ted into an organic solvent. Similarly, primary, secondary, and tertiary amines can be protonated with a strong acid to transfer the amine into a water solution, for example, as an amine hydrochloride salt. Conversely, a strong base can be added to convert the amine salt back to free base, which can be extracted into a solvent. This procedure is quite common in pharmaceutical production. 

Ion-exchange resins swell in water to an extent which depends on the amount of crosslinking in the polymer, so that columns should be prepared from the wet material by adding it as a suspension in water to a tube already partially filled with water. (This also avoids trapping air bubbles.) The exchange capacity of a resin is commonly expressed as mg equiv./mL of wet resin. This quantity is pH-dependent for weak-acid or weak-base resins but is constant at about 0.6-2 for most strong-acid or strong-base types. 

In addition to the formulation parameters mentioned above, selection of the base used for catalysis has strong implications. Bases commonly used are sodium hydroxide, potassium hydroxide, lithium oxide, calcium hydroxide, barium hy-... 

A base is any material that produces hydroxide ions when it is dissolved in water. The words alkaline, basic, and caustic are often used synonymously. Common bases include sodium hydroxide (lye), potassium hydroxide (potash lye), and calcium hydroxide (slaked lime). The concepts of strong versus weak bases, and concentrated versus dilute bases are exactly analogous to those for acids. Strong bases such as sodium hydroxide dissociate completely while weak bases such as the amines dissociate only partially. As with acids, bases can be either inorganic or organic. Typical reactions of bases include neutralization of acids, reaction with metals, and reaction with salts ... 

The two most common elimination reactions arc dehydroUalogenalion—the loss of HX from an alkyl halide—and dehydration—(he loss of water from an alcohol. Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base such as potassium hydroxide. For example, bromocvclohexane yields cyclohexene when treated with KOH in ethanol solution. 

The E2 reaction (for elimination, bimolecular) occurs when an alkyl halide is treated with a strong base, such as hydroxide ion or alkoxide ion (RO-). It is the most commonly occurring pathway for elimination and can be formulated as shown in Figure 11.17. 

Enolate ions are more useful than enols for two reasons. First, pure enols can t normally be isolated but are instead generated only as short-lived intermediates in low concentration. By contrast, stable solutions of pure enolate ions are easily prepared from most carbonyl compounds by reaction with a strong base. Second, enolate ions are more reactive than enols and undergo many reactions that enols don t. Whereas enols are neutral, enolate ions are negatively charged, making them much belter nucleophiles. As a result, enolate ions are more common than enols in both laboratory and biological chemistry. 

Discussion. The hydroxides of sodium, potassium, and barium are generally employed for the preparation of solutions of standard alkalis they are water-soluble strong bases. Solutions made from aqueous ammonia are undesirable, because they tend to lose ammonia, especially if the concentration exceeds 0.5M moreover, it is a weak base, and difficulties arise in titrations with weak acids (compare Section 10.15). Sodium hydroxide is most commonly used because of its cheapness. None of these solid hydroxides can be obtained pure, so that a standard solution cannot be prepared by dissolving a known weight in a definite volume of water. Both sodium hydroxide and potassium hydroxide are extremely hygroscopic a certain amount of alkali carbonate and water are always present. Exact results cannot be obtained in the presence of carbonate with some indicators, and it is therefore necessary to discuss methods for the preparation of carbonate-free alkali solutions. For many purposes sodium hydroxide (which contains 1-2 per cent of sodium carbonate) is sufficiently pure. 

Now consider strong and weak bases. The common strong bases are oxide ions and hydroxide ions, which are provided by the alkali metal and alkaline earth metal oxides and hydroxides, such as calcium oxide (see Table J.l). As we have seen,... 

The cyclopentadienide ion, C H , is a common organic anion that forms very stable complexes with metal cations. The anion is derived by removing a proton from cyclopentadiene, QH, with strong base. The molecule has a five-memhered ring of carbon atoms, with four carbon atoms attached to only one proton and one carbon atom bonded to two. Draw the Lewis... 

As for acids, the strength of a base depends on the solvent a base that is strong in water may be weak in another solvent and vice versa. The common strong bases in aqueous solution are listed in Table J.l. 

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