Hydrogen bonding pdf.pdf

1.Weak forces of attraction
2.Concepts of Hydrogen bonding
3.Types of hydrogen bonding
4.Properties of hydrogen bond.
5.Methods of detection of hydrogen bond.
6.Importance of Hydrogen bonding.
7.Vander walls forces
a.Ion-dipole
b.Dipole-dipole
c.London forces.
8.Origin of hydrogen bonds.
9.Consequences of hydrogen bonding.
10.Ice has less density than water.
11.Intermolecular forces.

1. Dr.V.A.Thakur
Deptt.ofChemistry
K.B.P.College,Vashi
(Empowered
Autonomous)
Navi Mumbai
Hydrogen Bonding

2. Points to be covered
➢Weak forces of attraction
➢Hydrogen bonding
➢Concepts
➢Types
➢Properties
➢Methods of detection and importance
➢Vander walls forces
➢Ion-dipole
➢Dipole-dipole
➢London forces

3. Weak forces of Attractions
Chemical forces and Interactions :
Types of InteractionStrength Energy –distance function
Covalent Bond Verystrong Complex, but comparatively
long range
Ionic Bond Verystrong 1/r, comparatively long range
Ion-dipole Strong 1/r
2
, short range
Dipole-dipole Moderately strong1/r
3
, short range
Ion-induced dipoleweak 1/r
4
, very short range
Dipole-induced dipoleVery weak 1/r
6
, extremely short range
London dispersion
forces
Very weak 1/r
6
, extremely short range

4. Hydrogen Bond
Incompoundsofhydrogenwithstronglyelectronegativeelements,
suchasfluorine,oxygenandnitrogen,theelectronpairsharedbetween
twoatomsliesfarawayfromhydrogenatom.
Thisresultsintohydrogenatombecomeselectropositivewithrespectto
otheratom.
ThisphenomenonofchargeseparationinHydrogenfluorideis
representedasH
+
--F
-
.
Suchamoleculeissaidtobepolar.
Themoleculebehavesasadipolebecauseoneendcarriesapositive
chargeandotherendanegativecharge.
Theelectrostaticforceofattractionbetweensuchmoleculesshouldbe
strong.
Becausepositiveendofonemoleculeattractsandisattractedbythe
negativeendofothermolecule.
Thus,twoormoremoleculesmayassociatetogethertoformlarge
clustersofmolecules.

5. ...H
+
-F
-
...H
+
–F
-
....H
+
-F
-
...H
+
-F
-
....
TheclusterofHFmoleculesmaybedescribedas
(HF)n.
Theattractiveforcewhichbindshydrogenatomof
onemoleculewithelectronegativeatom(suchas
fluorine)ofanothermolecule,generallyofthesame
substanceisknownasHydrogenBond.
Inthiscase,thelengthofthecovalentbondbetweenF
andHatomsisfoundto1.00A
0
andthelengthof
hydrogenbondbetweenFandHatomsofneighbouring
moleculeisfound1.55A
0

6. Hydrogen bonds
Watermoleculesbecauseofitsbentstructure,isalsoa
dipoleasoxygenendcarryinganegativechargeand
hydrogenendcarryingapositivecharge.Hydrogen
bondinginthismoleculecantakesplace.
Theclusterofwatermoleculescanberepresentedas
(H
2O)
n
Alcoholsandcarboxylicacidsalsoformassociated
molecules.
Inwaterandalcohols,thehydrogenbonding(associationof
molecules)mayextendtoseveralmolecules.
Butincarboxylicacids,thehydrogenbondingislimitedto
associationoftwomoleculesonly.

7. Nature of Hydrogen bond
Thehydrogenbondisaclassinitself.
Itarisesfromelectrostaticforcesbetweenthepositiveend(pole)
ofonemoleculeandthenegativeend(pole)ofanothermolecule
ofthesameorsomeotherpolarsubstance.
E,g.Extensivehydrogenbondingoccursbetweenmoleculesof
water,moleculesofethanolandalsobetweenmoleculesofwater
andmoleculesofethanol.
Thestrengthofhydrogenbondisfoundtovarybetween
10
-
40KJpermole(i,e.per6.022x10
23
bonds.)
Thatofcovalentbondhasfoundtobeoftheorderof400KJ
mole.
Thus,ahydrogenbondisverymuchweakerthanacovalent
bond.

9. The Origin of Hydrogen bonds
Hydrogen: attached
directly to one of the most
electronegative elements
Electronegative elements:
not only significantly
negative charged, but also
has at least one "active"
lone pair.
Electrons: contained in a
relatively small volume of
space which therefore has
a high density of negative
charge.

11. Inter-molecular
Bonding
Intra-molecular
COVALENTHydrogen bond
Van der Waals
Etc.
,, …
Dipole-dipole
Dipole-Induced dipole
Instantaneous dipole-induced dipoleLondon Dispersion
Relative strengths
dispersion forces < dipole-dipole interactions < hydrogen bonds
Ion-dipole
Cation-Pi
Pi-Pi
The term ‘Van der Waals forces’ is sometimes used for a specific type (London Dispersion) rather than the class
We will describe briefly a few of these (only) here

12. Types of Hydrogen bonding
▪Thecovalentbodingbetweenahydrogen
atomandastronglyelectronegativeatom
becomes‘polar’-covalent
▪The‘charged’hydrogen‘ion’canbe
attractedtoaelectronegativeatom,suchas
nitrogen,oxygenorfluorine
▪hydrogenbondshouldnotbeconfused
withacovalentbondtohydrogen.
▪Types of hydrogen bonds:
➢Intermolecular(betweenmolecular)
➢Intramolecular(withinamolecule)
▪E.g.ofhydrogenbonding:water
(responsibleforthehighboilingpointof
watercomparedtosayH
2S),DNA,partly
responsibleforthesecondary,tertiary,and
quaternarystructuresofproteinsand
nucleicacids,Polymers
O
−
H
+ H+
O
−
H
+ H+
Hydrogen bond

13. Electronegativityis the tendency of an atom in a molecule to attract shared electrons to
itself. An electronegative atom pulls more of the electron density from the bond towards
itself.

16. A hydrogen bond is the attractive force between one
electronegative atom and a hydrogen covalently
bonded to another electronegative atom.
It results from a dipole-dipole force with a hydrogen
atom bonded to nitrogen,oxygenor fluorine (thus the
name "hydrogen bond", which must not be confused
with a covalent to hydrogen).
The energy of a hydrogen bond (typically 5 to 30
kJ/mole) is comparable to that of weak covalent bonds
(155 kJ/mol), and a typical covalent bond is only 20
times stronger than an intermolecular hydrogen bond.

17. These bonds can occur between molecules (intermolecularly), or
within different parts of a single molecule (intramolecularly).
The hydrogen bond is a very strong fixed dipole-dipole Vander
Walls-Keesomforce , but weaker than covalent, ionic and metallic
bonds.
The hydrogen bond is somewhere between a covalent bond and an
electrostatic intermolecular attraction.
This type of bond occurs in both inorganic molecules (such as
water) and organic molecules (such as DNA).
Intermolecular hydrogen bonding is responsible for the high
boiling point of water (100 °C).
This is because of the strong hydrogen bond, as opposed to other
group16 hydrides.
Intramolecularhydrogen bonding is partly responsible for the
secondary, tertiary and quaternary structures of proteins and
nucleic acids.

18. Properties of Hydrogen Bond
❖1)Itisabondbetweentwoelectronegativeatomsonly.Itneverinvolves
morethantwoatoms(excludingHatoms).
❖2)BondEnergyofaH-bondisintherangeof3-10Kcal/mole.While
thatofanormalcovalentbondisintherangeof50-100Kcal/mole.
ThusaH-bond(H---B)ismuchweakerthanacobalentbondA-H.
❖ThedifferenceinenergybetweenA-HandH...Bbondindicatesthey
havedifferentbondlengthwhichinturnshowsthatHatominA-H...B
isnevermidwaybetweentwoatomsAandB.
❖ItisratheralwaysnearertoatomAwhichiscovalentlybondedtoH-
atom.
❖H-Bondhasmoreenergy(=3-10Kcal/mole)thanVanderWallsforces(=
1Kcal/mole).
❖3)TheformationofH-bonddoesnotinvolveanysharingofelectron
pairs.Itisthereforequitedifferentfromacovalentbond.

19. Properties of Hydrogen Bond
4)H-bondinA-H....BisformedeasilywhenboththeatomsA&
Barehighlyelectronegative.
ThustheeaseofformationofH-bondinA-H...Bincreseswith
theincreasesintheelectronegativityvalueofatomAindicating
as
A.

ThistendencyclearlyexplainsthattendencyofA-Hbondto
formaH-bondincreasesfromN-HthroughO-HtoF-Has

N
O
F.
ThistendencydecreasesinpassingfromO-HtoS-HorfromF-
HtoCl-Hbecause
O
sand
F
Cl
ThisshowsthatFatomwithhighestelectronegativity(
F)forms
thestrongerH-bond.

20. Formation of hydrogen bonds between H
2O molecules.
hydrogen bond
Electrostatic attraction exists between partial positive charge of H
atom and the lone pair electrons of O atom of another H
2O.

21. Formation of hydrogen bonds between NH
3
molecules
hydrogen bond
Electrostatic attraction exists between partial positive charge of H
atom and the lone pair electrons of N atom of another NH
3.

22. Consequences of Hydrogen Bonding
Properties explained by Hydrogen bonding:
1. State of H
2
O and H
2 S:
TheeaseofformationofH-BondinA-H...Bdecreaseswiththe
decreaseintheelectronegativityofatomB.
Thusas
O
s,ThereisaconsiderableHydrogenbonding
inH
2OwhileinH
2S,itisabsent.
Inotherwords,H
2Omoleculecanassociatetogetherto
formapolymerizedmolecule,(H
2O)n(calledcluster)in
whichhydrogenatomactsasabridgebetweentwooxygen
atomswhicharehighlyelectronegative.
Duetoformationofthispolymerized(H
2O)
nmolecule
containingHydrogenbondswaterexistsasaliquid.
InH
2SthereisnoHydrogenbondformation.Henceitdoes
notformcluster.Henceitexistsasagas.

23. Melting and Boiling points of Hydrides of N,O & F
If the melting points and boiling points of the Hydrides
of the elements of Group IV A, V A, VI A & VIIA are
plotted against their molecular weights of these
hydrides, we get following plot.
Group 15
Formula Molar mass B.P.(0C)
Group 16
Formula Molar mass B.P. (0C)
Group 17
Formula M.MassB.P.(.(0C)
SbH
3 125 -17.0 H
2Te 130-1.8HI127.9-3.5
AsH
3 78 -55.1 H
2Se 81 -42.0HBr80.9-67.1
PH
3 34 -84.6 H
2S 93 -59.6HCl36.5-85.0
NH
3 17 -33.0 H
2O 18 100 HF200 +19.4

24. Where does H bond come from?

25. Althoughtheboilingpointsineachgroupdecreaseswith
decreaseinmolarmass,thereissuddenreversalincaseof
Ammonia,WaterandHydrogenfluorideinGroup15,16&17
respectively.
Theunusualhighboilingpointsofeachcompoundisa
consequenceofstrongintermolecularforcesduetoH-bonding.
ThesuddenincreaseinM.P.andB.P.Inthesehydridesisdueto
inter-molecularH-bondinginbetweenHandNinNH
3.
TheexistenceofH-bondinginthesemoleculesgives
polymerizedmolecule(NH
3)n,(H
2O)nand(HF)nhavingH-
bond.
TobreaktheH-bondexistinginthesepolymerizedmoreenergy
isrequired.
ThusM.P.&B.P.ofthesemoleculesissuddenlyraised.

26. H –bonding and boiling pointPredicted and actual boiling points
-200
-150
-100
-50
0
50
100
Period
Boiling point
Group 4
Group 5
Group 6
Group 7
2 3 4 5

27. Ice has less Density than water
Inthecrystalstructureofice,theoxygenatomissurroundedbyfourH-
atoms.
TwoH-atomsarelinkedtoO-atombycovalentbondsandthe
remainingtwoH-atomsarelinkedtoO-atombytwoH-bondsshown
bydottedline.
Thus,inice,everywatermoleculeisassociatedwithfourotherwater
moleculesbyH-bondinginatetrahedralfashion.
IcehasopenstructurewithalargeemptyspaceduetoexistenceofH-
bonds.
Asicemeltsat0
0C,
numberofH-bondsarebrokendownandthespace
betweenwatermoleculesdecreasessothatwatermoleculesmoveclose
together.
Thedensityofwaterthereforeincreasesfrom0
0C
to4
0C
atwhichitis
maximum.

28. a water
molecule
hydrogen
bond
hydrogen atom
oxygen atom

29. Above4
0C
,theincreaseiskineticenergyofthemoleculeissufficient
tocausethemoleculestobegintodisperse.
Thisresultintosteadydecreaseindensityasthetemp.increases.
4.Thereisacontractioninwaterwhenitiswarmedupto4
0C.
Aswateriswarmedfrom0
0C
onwardsmoreandmoreHydrogenbonds
breakdownsothatwatermoleculescomecloserandclosertoone
another.
Themovingofwatermoleculesclosertogetherresultsincontraction.
Thereisalsoanexpansioninthevolumeofwater.
Itappearsthatupto4
0C
theeffectofexpansionpredominates.
Hencethereiscontractioninthevolumeofwaterwhenitiswarmed
upto4
0C
Above4
0C
theeffectofexpansionpredominatesandhencethereisan
expansioninthevolumeofwaterwhenitisheatedatatemperature
morethan4
0C
.

30. Methods of Detection of Hydrogen Bond
ThemethodshavebeenusedforthedetectionofHydrogen
Bondindifferentcompounds.
1.InfraRedSpectroscopy:
WhenacomplexofthetypeA-H..BcontainingHydrogen
bondisformed,severalfollowingchangesareobservedin
theinfra-redregion.
A)TheabsorptionbandsduetotheA-Hstretching
vibrations(fundamentalandovertones)areshiftedtolower
frequencies.
Theseshiftsrangefromabout30cm
-1
toseveralhundreds
cm
-1
ormore.
Thisshiftingisduetotheweakeningoftheforceconstant
forA-H.

31. Methods of Detection of Hydrogen Bond
❑Infrared and Raman Spectroscopy
❑Gas-Phase Microwave Rotational Spectroscopy
❑Neutron Inelastic Scattering
❑NMR Spectroscopy
❑Deuteron Quadruple Coupling
❑Diffraction Methods: Neutron and X-Ray
❑Computational Chemistry
❑ThermochemicalMethods

32. Importance of Hydrogen Bonding
➢WithoutHydrogenbonding,waterwouldhaveexistedasagas
likeHydrogenSulphide(H
2S).Hencenolifewouldhavebeen
possibleonthisglobe.
➢Hydrogenbondingalsoexistsinalllivingorganismofanimalor
ofvertebratekingdom.Thusitexistsinvarioustissues,organs,
blood,skinandbonesinanimallife.
➢Itplaysanimportantroleindeterminingthestructureofproteins
whicharesoessentialforlife.
➢Hydrogenbondingplaysanimportantroleinmakingwood
fibresmorerigid.Thusitmakesanarticleofgreatutilitytomeet
requirementsofhousing,furnitureetc.
➢Thecotton,silkorsyntheticfibresowetheirrigidityandtensile
strengthtoHydrogenbonding.ThusH-bondingisofvital
importanceforourclothingalso.

33. Importance of Hydrogen Bonding
Mostofourfoodmaterialsalsoconsistof
hydrogenbondedmolecules.e,g.Sugarsand
Carbohydrateshavemany-OHgroups.The
Oxygenofonesuchgroupinonemoleculeis
bondedwithOHgroupofanothermolecule
throughH-bonding.
Thethicknessofglue(aprotein)orhoney(which
consistsmainlyofwaterandsugar)isalsodueto
Hydrogenbondingbetween-OHorothersuch
groupsofdifferentmoleculeswithoneanother.

36. What hydrogen bonds help to do?
Multiple hydrogen bonds.
➢hold the two strands of the DNA double helix
together .
➢hold polypeptidestogether in such secondary
structures as the alpha helix and the beta
conformation
➢helps enzymesin bind to their substrates
➢helps antibodiesbind to their antigen
➢helps transcription factorsbind to each other and
DNA ……

37. H-bond in complex molecules

38. A Special Molecule: H
2O

40. Intermolecular forces
Theforcesofattractionexistsbetweenpolaraswellasnon-polar
moleculesareknownasIntermolecularforcesorCohesiveforcesor
VanderWaalsforces.
Theseforcesoriginatefromtwotypesofinteractions.
1.Dipole-dipoleinteractions.
2.InducedDipole-induceddipoleinteractions,
1.Dipole-DipoleInteractions:
Incaseofpolarmoleculeswhichhavepermanentdipoles,theVander
Waalsforcesaremainlyduetoelectricalinteractionbetweenthe
dipolesknownasdipole-dipoleinteraction.
Gasessuchasammonia,Sulphurdioxide,hydrogenfluoride,hydrogen
chlorideetc.haveapermanentdipolesasaresultofappreciabledipole-
dipoleinteractionsbetweenthemoleculesofthesegases.
Themagnitudeofthistypeofinteractiondependsonthedipole
momentofthemoleculeconcerned.

41. Thegrateristhedipolemoment,greateristhedipole-
dipoleinteractions.
Becauseoftheattractiveinteractions,thesegasescanbe
easilyliquefied.
Theaverageinteractionenergyofthetwomoleculeswith
permanentdipolemoment
1
and
2
isgivenby
InteractionEnergy((r)=
2[
1

2
/4
0
]
2
(1/r6)(1/3KT)
Wherer=distancebetweenthemolecules
K=Boltzmanconstant
4
0
=Permitivityfactorforthemedium

42. Induced dipole-Induced dipole interaction, London forces or
Dispersive forces
WeunderstandthatVanderWaalsforcesexisteveninnon-polar
diatomicmoleculessuchasO
2
andN
2
.
ItisalsopresentinNon-polarmonoatomicmoleculessuchas
He,NeandAretc.
Thisattractionisevidentfromthecondensationofthesegases
intoliquidsofsufficientlyhighpressureandlowtemperature.
TheexistenceofVanderWaalsforcesinthesemoleculescould
notunderstoodforseveralyears.
In1930,F.Londonprovidesasatisfactoryexplanationforthe
existenceofforcesofattractionbetweennon-polarmolecules.
Accordingtowhichelectronsofanon-polarmoleculeskeepon
oscillatingwithrespecttonucleiofatoms.
Asaresultofthis,atgiveninstant,positivechargemaybe
concentratedinoneregionandnegativechargeinanotherregion
ofthesamemolecule.

43. Thusanon-polarmoleculehasbecomemomentarilyself-
polarised.
Thispolarisedmoleculemayinducedipolemomentina
neighbouringasshowninfigure.
Theelectrostaticforcesofattractionbetweeninduced
dipolesandoriginaldipoles(duetooscillationofelectrons)
areknownasLondonForces.
Theseforcesarealsocalledasdispersiveforcesbecause
thewellknownphenomenonofdispersionoflightisalso
connectedwiththesedipoles.
Forapairofadjacentmolecules,Londonforcesvary
inverselyasthesixthpowerofthedistancebetweenthem.
F1/r6

44. The approximate interaction energy in this case is given by
(r) = ---([3 E
1
E
2
/2(E
1
+ E
2
)] [ 
1

2
/ (4
0
]
2
(r6)
Where
E
1
,E
2
= Ionization energies of two molecules.
Other parameters have same significance.
The Vander Waals attraction in non-polar molecules is thus
exclusively due to London forces.

45. Van derWaals
Dipole-Dipole interactions
▪In the covalent bonding between two atoms
of very different electronegativitythe bond
becomes highly polar (introducing partial
charges on the species)
▪This dipole can interact with other
permanent dipoles
▪This interaction is stronger than dispersion
forces
Br
+
F
− Br
+
F
−

46. Instantaneous dipole-
induced dipoleLondon Dispersion
▪Instantaneously generated dipole (due to asymmetry in electron charge
distribution around the nucleus) on one atom leads to slight polarization of
the atom
(→ quantum induced instantaneous polarization)
➢This induces a dipole on the neighbouringatom (temporarily)
▪The force between these two dipoles is called the London dispersion
forces
▪The force is very weak and is temporally varying
▪Can operate between non-polar molecules (H
2, Cl
2, CO
2etc.)
▪The strength of the dispersion forces will increase with number of
electrons in the molecule
Ar
Ar

47. London forces
Induced dipole
A dipole forms in one atom or
molecule, inducing a dipole in the
other
Eventually electrons are situated so that
tiny dipoles form

48. Ion-Dipole
▪Permanent dipole
interacts with an ion.
▪This explains for
example the solubility
of NaClin water.
▪The figure below shows
the interaction of Na
+
and Cl
−
ions interacting
with the permanent
dipoles in a water
molecule.