1.3- Butanediols

HOCH2CH2CH2CH2OH. B.p. 228"C. Prepared ethyne plus methanal, hydrogenated to butanediol. Used in production of y-buty-rolactone and 2-pyrrolidone. Widely used in polyurethane products, butylenes See butenes. 

Note. Both tetramethylene glycol (1 4-butanediol) and hexamethylene glycol (1 6 hexaiiediol) may be prepared more conveniently by copper-chromium oxide reduction (Section VI,6) or, for small quantities, by reduction with lithium aluminium hydride (see Section VI,10). 

Dichlorobutane. Place 22-5g. of redistilled 1 4-butanediol and 3 ml. of dry pyridine in a 500 ml. three necked flask fitted with a reflux condenser, mechanical stirrer and thermometer. Immerse the flask in an ice bath. Add 116 g. (71 ml.) of redistilled thionyl chloride dropwise fix>m a dropping funnel (inserted into the top of the condenser) to the vigorously stirred mixture at such a rate that the temperature remains at 5-10°. When the addition is complete, remove the ice bath, keep the mixture overnight, and then reflux for 3 hours. Cool, add ice water cautiously and extract with ether. Wash the ethereal extract successively with 10 per cent sodium bicarbonate solution and water, dry with anhydrous magnesium sulphate and distil. Collect the 1 4-dichloro-butane at 55-5-56-5°/14 mm. the yield is 35 g. The b.p. under atmospheric pressure is 154 155°. 

Dibromobutane from 1 4 butanediol). In a 500 ml. threenecked flask fltted with a stirrer, reflux condenser and dropping funnel, place 154 g. (105 ml.) of 48 per cent, hydrobromic acid. Cool the flask in an ice bath. Add slowly, with stirring, 130 g. (71 ml.) of concentrated sulphuric acid. To the resulting ice-cold solution add 30 g. of redistilled 1 4-butanediol dropwise. Leave the reaction mixture to stand for 24 hours heat for 3 hours on a steam bath. The reaction mixture separates into two layers. Separate the lower layer, wash it successively with water, 10 per cent, sodium carbonate solution and water, and then dry with anhydrous magnesium sulphate. Distil and collect the 1 4-dibromo-butane at 83-84°/12 mm. The yield is 55 g. 

Dibromobutane (from 1 4-butanediol). Use 45 g. of redistilled 1 4-butanediol, 6-84 g. of purified red phosphorus and 80 g. (26 ml.) of bromine. Heat the glycol - phosphorus mixture to 100-150° and add the bromine slowly use the apparatus of Fig. Ill, 37, 1. Continue heating at 100-150° for 1 hour after all the bromine has been introduced. Allow to cool, dilute with water, add 100 ml. of ether, and remove the excess of red phosphorus by filtration. Separate the ethereal solution of the dibromide, wash it successively with 10 per cent, sodium thiosulphate solution and water, then dry over anhydrous potassium carbonate. Remove the ether on a water bath and distil the residue under diminished pressure. Collect the 1 4-dibromobutane at 83-84°/12 mm. the yield 3 73 g. 

In a 500 ml. three-necked flask, equipped with a thermometer, a sealed Hershberg stirrer and a reflux condenser, place 32-5 g. of phosphoric oxide and add 115-5 g. (67-5 ml.) of 85 per cent, orthophosphoric acid (1). When the stirred mixture has cooled to room temperature, introduce 166 g. of potassium iodide and 22-5 g. of redistilled 1 4-butanediol (b.p. 228-230° or 133-135°/18 mm.). Heat the mixture with stirring at 100-120° for 4 hours. Cool the stirred mixture to room temperature and add 75 ml. of water and 125 ml. of ether. Separate the ethereal layer, decolourise it by shaking with 25 ml. of 10 per cent, sodium thiosulphate solution, wash with 100 ml. of cold, saturated sodium chloride solution, and dry with anhydrous magnesium sulphate. Remove the ether by flash distillation (Section 11,13 compare Fig. II, 13, 4) on a steam bath and distil the residue from a Claisen flask with fractionating side arm under diminished pressure. Collect the 1 4-diiodobutane at 110°/6 mm. the yield is 65 g. 

Another possibility for asymmetric reduction is the use of chiral complex hydrides derived from LiAlH. and chiral alcohols, e.g. N-methylephedrine (I. Jacquet, 1974), or 1,4-bis(dimethylamino)butanediol (D. Seebach, 1974). But stereoselectivities are mostly below 50%. At the present time attempts to form chiral alcohols from ketones are less successful than the asymmetric reduction of C = C double bonds via hydroboration or hydrogenation with Wilkinson type catalysts (G. Zweifel, 1963 H.B. Kagan, 1978 see p. 102f.). 

Now think about a molecule such as 2 3 butanediol which has two chirality centers that are equivalently substituted... 

Only three not four stereoisomeric 2 3 butanediols are possible These three are shown m Eigure 7 10 The (2R 3R) and (2S 3S) forms are enantiomers of each other and have equal and opposite optical rotations A third combination of chirality centers (2R 3S) however gives an achiral structure that is superimposable on its (2S 3R) minor image Because it is achiral this third stereoisomer is optically inactive We call achiral mole cules that have chnahty centers meso forms The meso form m Eigure 7 10 is known as meso 2 3 butanediol... 

One way to demonstrate that meso 2 3 butanediol is achiral is to recognize that its eclipsed conformation has a plane of symmetry that passes through and is perpendicular to the C 2-C 3 bond as illustrated m Eigure 7 11a The anti conformation is achiral as... 

FIGURE 7 10 Stereo isomeric 2 3 butanediols shown in their eclipsed con formations for convenience Stereoisomers (a) and (b) are enantiomers of each other Structure (c) is a diastereo mer of (a) and (b) and is achiral It is called meso 2 3 butanediol... 

In the same way that a Fis cher formula is a projection of the eclipsed conformation onto the page the line drawn through its center is a projection of the plane of symmetry that is present in the eclipsed conformation of meso 2 3 butanediol... 

Fischer projection formulas can help us identify meso forms Of the three stereoisomeric 2 3 butanediols notice that only in the meso stereoisomer does a dashed line through the center of the Fischer projection divide the molecule into two mirror image halves... 

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