Synthesis of Optically Active Compounds
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What often happens when optically active compounds are produced in organic synthesis? (rctn. prdcs. 50|50 mxtr. 2OI)
The reaction produces a 50:50 mixture of two optical isomers
What is a 50:50 mixture of two optical isomers called?
It is called a racemic mixture or racemate
Why is a racemic mixture not optically active? (efcts. 2 ismrs. plne. plrsd. lgt. cncl. echthr. out)
Because the effects of the two isomers on plane - polarised light cancel each other out
Outline the nucleophilic addition reaction between hydrogen cyanide (HCN) and ethanal to form 2- hydroxypropanenitrile -> Mark the chiral molecule
Check kerboodle answer
What type of reaction is the addition of HCN to ethanal?
A nucleophilic addition reaction
What is the nucleophile in this reaction?
The CN- ion
Why does 2-hydroxypropanenitrile have optical isomers?
Because they contain a chiral centre
Why does the nucleophilic addition of :CN across ethanal produce a racemic mixture? (CN atck. eqly. frm. A/B plnr. ethnl. mlcle. - bth. entmrs. frm. eql. amnts.)
The CN- group can attack equally from above or below the planar CH3CHO molecule, so both enantiomers form in equal amounts
What happens in stage 2 of the synthesis of lactic acid? (ntrle. grp. 2-hydroxypropanenitrile cnvrtd. crbxlc. acd. grp.)
The nitrile group in 2-hydroxypropanenitrile is converted into a carboxylic acid group
Outline the hydrolysis reaction where 2 - hhdroxypropanenitrile with a dilute solution of hydrochloric acid (HCL) to produce 2 - hydroxypropanoic acid Mark the chiral carbon in the molecule
Check the kerboodle page
What reagents are used to hydrolyse 2-hydroxypropanenitrile? (wtr. acdfd. w/ dlte. sol. HCL)
It is reacted with water acidified with a dilute solution of hydrochloric acid
What type of reaction is the conversion of 2-hydroxypropanenitrile to 2-hydroxypropanoicacid?
Hydrolysis reaction
What is the product of the hydrolysis of 2-hydroxypropanenitrile?
Lactic acid (2-hydroxypropanoic acid)
Does the hydrolysis affect the chiral centre in the molecule? (hydlys. chngs. CN grp. - chrl. cntr. unchngd.)
No, the hydrolysis only changes the -CN group, so the chiral centre is unchanged
What happens when a chiral molecule is produced naturally, that does not occur when produced synthetically? (prsnt. 1OI instd. 2)
It is often present as only one optical isomer, instead of two
What is the problem if only one optical isomer of a drug is effective? (othr. ismr. inactv. mkng. drg. lss. efcnt.)
The other isomer may be inactive, making the drug less efficient
What are the three options for dealing with inactive isomers in drugs? (sprte. 2 ismrs. - slng. rcmc. mxtr. - dsgng. alt. synth. oly. prdcs. rqrd. ismr)
• Separate the two isomers • Selling the racemic mixture • Designing an alternative synthesis that only produces the required isomer
What are the two disadvantages when separating two isomers and selling the racemic mixture? (sprtng. 2I - dfclt. + expnsve, slng. rcmc. mxtr. wld. wstfl. sne. hlf. inactve.)
Separating the two isomers, can be difficult and expensive. Selling the racemic mixture would be wasteful since half is inactive