3. NaBH4

Characteristics and uses of NaBH4 is discussed.

1. 3. NaBH
4
Dr. Shivendra Singh
UGC-NET-JRF, PhD-IIT Indore

2. …Sodium borohydride: Introduction
Itisalsoknownassodiumtetrahydridoborateandsodiumtetrahydroborate.
Thecompoundwasdiscoveredinthe1940sbyH.I.Schlesinger.
Itissolubleinproticsolventsandloweralcohols.
ItalsoreactswiththeseproticsolventstoproduceH
2;however,these
reactionsarefairlyslow.
NaBH
4
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3. …NaBH
4: Structure
NaBH
4isasalt,consistingofthetetrahedral[BH
4]
−
anion.
Thesolidisknowntoexistasthreepolymorphs:α,βandγ.
Thestablephaseatroomtemperatureandpressureisα-NaBH
4,whichis
cubicandadoptsanNaCl-typestructure.
Atapressureof6.3GPa,thestructurechangestothetetragonalβ-NaBH
4
andat8.9GPa,theorthorhombicγ-NaBH
4becomesthemoststable.
NaBH
4
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4. …NaBH
4: Structure
S. No.Crystal systemBond lengthInterfacial angle
1 Cubic a = b = c α = β = γ = 90°
2 Tetragonal a = b = c α = β = γ = 90°
3 Orthorhombica = b = c α = β = γ = 90°
NaBH
4

5. …NaBH
4: Synthesis
Commercial
NaBH
4production
Brown-Schlesinger
process
Bayer process
B(OCH
3)
3+ 4 NaH → NaBH
4+ 3 NaOCH
3
Industrially prepared from
sodium hydride and trimethyl
borate at 250–270 °C:
It is based on the reaction
among borax, Na, H
2, and
silicon oxide (SiO
2) at 700
°C.
Na
2B
4O
7+ 16 Na + 8 H
2+ 7 SiO
2→ 4 NaBH
4+ 7 Na
2SiO
3
NaBH
4
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6. …NaBH
4: Reactivity
Mosttypically,itisusedinthelaboratoryforconvertingketonesand
aldehydestoalcohols.
Itefficientlyreducesacylchlorides,anhydrides,α-hydroxylactones,
thioesters,andiminesatroomtemperatureorbelow.
Itreducesestersslowlyandinefficientlywithexcessreagentand/or
elevatedtemperatures,whilecarboxylicacidsandamidesarenot
reducedatall.
NaBH
4

7. …NaBH
4: Reactivity
Hydrogen-bondingactivationisrequired,asnoreductionoccursinan
aproticsolventlikediglyme.
α,β-UnsaturatedketonestendtobereducedbyNaBH
4ina1,4-sense,
althoughmixturesareoftenformed.
NaBH
4

8. …NaBH
4: Applications
1. Reduction of Aldehydes and Ketones with NaBH
4
NaBH
4

9. …NaBH
4: Mechanism (2 steps)
i.Inthefirststep,H
-
detachesfromtheBH
4
–
andaddstothecarbonylcarbon
([1,2]-addition).ThisformstheC-Hbond,andbreakstheC-Obond,resultingin
anewlonepairontheoxygen,whichmakestheoxygennegativelycharged
(alkoxides,astheyaredeprotonatedalcohols).
ii.Inthesecondstep,aprotonfromwater(oranacid)isaddedtothealkoxideto
makethealcohol.Thisisperformedattheendofthereaction,astepreferredto
astheworkup.
NaBH
4

10. …NaBH
4: Reactions
2.For“Demercuration”(TheSecondStepOfOxymercuration-
Demercuration):Itisusedinthesecondstep,tobreaktheC-Hgbondand
turnitintoaC-Hbond.
NaBH
4

11. …NaBH
4: Reactions
NaBH
4

12. …NaBH
4: Reactions
https://www.chemistrylearner.comNaBH
4

13. …NaBH
4: Reactions
NaBH
4

14. …NaBH
4: Reactivity
α,β-UnsaturatedketonestendtobereducedbyNaBH
4ina1,4-sense,
althoughmixturesareoftenformed.
Additionofceriumchlorideasanadditivegreatlyimprovesthe
selectivityfor1,2-reductionofunsaturatedketones(Luchereduction).
α,β-Unsaturatedestersalsoundergo1,4-reductioninthepresenceof
NaBH
4.
NaBH
4

15. …NaBH
4: Luche reduction
Luche Reduction (NaBH
4+ CeCl
3):
A. L. Gemal, J. L. Luche, Journal of the American Chemical Society 1981,103,5454
CeCl
3+ NaBH
4 HCeCl
2
Theceriumreagentcoordinatesto
thecarbonyl,makingonlya1,2
additionpossible.
NaBH
4

16. …NaBH
4: Luche Reduction
MODERN METHODS OF ORGANIC SYNTHESIS, W. CARRUTHERS , IAIN COLDHAM NaBH
4

17. LAH vs NaBH
4
S.
No.
Property LAH NaBH4
1 Molar Mass 37.95 g/mol 37.83 g/mol
2 M.P. 150 °C 400 °C
3 RecrystallizationDiethyl ether is
used
Warm diglyme is used
4 Reaction with
water
Violently Slowly
5 Reducing powerStrong; Ester,
amide, carboxylic
acids etc.
Mild; Aldehydes, ketones, acyl
chloride, thiols, imines but not
Ester, amide and carboxylic acids.
NaBH
4

18. LAH vs NaBH
4
Jonathan Clayden (University of Manchester), Nick Greeves (University of Liverpool), Stuart Warren (University of Cambridge);Organic Chemistry, 2
nd
edition.NaBH
4

19. The order of strength is; LiAlH4 >NaBH4
i.Thecation:Thelithiumisbetterabletoactasalewisacidthanthesodium,
thelewisacidbondstothecarbonyloxygen.Thisinturnincreasesthe
amountofpositivechargedensityonthecarbonylcarbon.Asthereduction
isfavouredbyanincreaseinthepositivecharacterofthecarbonylcarbona
changefromsodiumtolithiumwillmakethereducingagentbetter.
ii.TheelectronegativityoftheatomatthecentreoftheEH
4anion:The
moreelectronegativetheatomisthelesselectrondensitywillbeonthe
hydrides,thelesselectrondensityofthehydridesthelessabletheyareto
actasnucleophilestoreducethecarbonyl.
{Pauling electronegativity of B is 2.04& of Al is 1.61}
NaBH
4

20. LAH vs NaBH
4
NaBH
4ispreferredforaldehydesandketonesbecauseitdoesnotreactviolently
withH
2O,thewayLAHdoesandcanbeusedasanaqueoussolution,whereasthe
LiAlH
4mustbedeliveredinananhydroussolutionofdiethyl-ether,andthen
neutralizedbywaterandacidtoisolatetheproduct/s.But,ultimately,LAHcanbe
usedforallofthesereactions.
https://www.quora.com
Carboxylicacidsandestersaremuch
lessreactivetoreductionthanare
ketonesandaldehydesandNaBH
4(aq)
istooweakareducingagentforthem.
NaBH
4

21. LAH vs NaBH
4
NaBH
4

22. …NaBH
4: SUMMARY
1.Introduction
2.Preparation
3.Reactivity & mechanism
4.Applications
i.Carbonyls
ii.Demercuration
iii.Luche reduction
5.Comparison with LAH
NaBH
4

23. Next: Wilkinson’s catalyst
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