THEORY NOTES ON CHEMICAL BONDING-II .pdf
0 likes
views
H bonding
THEORY NOTES ON CHEMICAL BONDING-II .pdf
0 likes
views
CHEMISTRY Theory Notes on Chemical Bonding-VI H- Bonding The idea of H-bonding was introduced to explain the nature of association in liquid state of Substances like water, HF, NH3, etc. In a hydrogen compound, when hydrogen is bonded to highly electronegative atom (such as F, O, N) by a covalent bond, the electron pair is attrached towards electronegative atom (such as F, O, N) by a covalent bond, the electron pair is attracted towards electronegative atom so strongly that a dipole results.If a member ofsuch molecules are brought nearer to each other, the positive end of one molecule and negative end of other molecule will attract each other and weak electro- static force will develop. Thus, these molecules will associate together to form a cluster of molecules. X – H ------- X – H ------ X – H ------- X – H ------- X – H Here the attractive force that binds hydrogen atom of one molecule with electronegative atom of the other molecule of the same or different substance is known as hydrogen bond. H-bonding is of two types- (a) Intermolecular H-bonding: This type of bonding results between positive ends negative ends of different atom of the same or different molecules. This type of H-bonding increases the boiling point of the compound and also its solubility in water due to association of several molecules of compound. Inter-molecular H-bonding is also responsible for increase in the viscocity. CH3 - CH2 - OH CH2 - OH | CH2 - OH | η = 1.2 CH2 - OH CH - OH | η = 17 CH2 - OH η = 107 This type of H-bonding takes place between H and electronegative element present in the different molecules of the same substance (as in between H2O and H2O) or different substances (as in between H2O and NH3) (A) In water molecules: Due to polar nature of H2O, there is association of water molecules giveing a liquid state of abnormally high b.p.: 2 2 2 2 H – O .... H – O .... H – O H – O | | | | Hδ Hδ Hδ Hδ If we actually observe, one water molecules is joined to four water molecules- two with H-atoms and other two with O-atom. Thus coordination number of water molecule in water is four. (B) In HF there is again linear chain association by H-bonding However in the gaseous state, severalpolymeric forms of the HF molecules exist in which the monomers are held together through H-bonding. It exist as (HF)6 (cyclic), (HF)5 (cyclic), (HF)2 (diner) upto and extent (HF, monomer) (C) Carboxylic acid dimerises is gaseous state due to H-bonding (D) Alcohol is said to be highly soluble in water due to crossed intermolecular H-bonding (between H2O and R-OH molecules). R H R | | | .... O – H .... O – H O – H δ δ δ δ δ δ However isomeric ether is less soluble in water due to its (ether) non-polar nature. δ δ δ CH3–CH2– O – H CH3– O –CH3 polar less polar Intra-Molecular H-bonding- This type of bonding results between hydrogen & an electronegative element both present in
THEORY NOTES ON CHEMICAL BONDING-II .pdf
- H- Bonding The ideaofH-bonding was introducedto explain the natureofassociationin liquid state ofSubstances like water, HF, NH3 , etc. In a hydrogen compound, when hydrogen is bonded to highly electronegative atom (suchas F, O,N) bya covalent bond, the electron pair is attrached towards electronegative atom(such as F, O, N) by a covalent bond, the electron pair is attracted towards electronegative atomso stronglythat a dipoleresults.Ifa member ofsuchmolecules are brought nearer to eachother, the positive end ofone molecule and negative end ofother molecule willattract each other and weak electro- static force willdevelop. Thus, these molecules willassociate together to formacluster ofmolecules. X – H ------- X – H ------ X – H ------- X – H ------- X – H Here the attractive force that binds hydrogen atomofone molecule with electronegative atomof the other molecule ofthe same or different substance is knownas hydrogenbond. H-bonding is of two types- (a) IntermolecularH-bonding: This type of bonding results between positive ends negative ends of different atom of the same or different molecules. Thistype ofH-bondingincreases the boilingpoint ofthecompoundandalso its solubilityinwaterdue to associationofseveralmolecules ofcompound. Inter-molecular H-bonding is also responsible for increase in the viscocity. CH3 - CH2 - OH CH2 - OH CH2 - OH | | η = 1.2 CH2 - OH CH - OH | η = 17 CH2 - OH η = 107 This type of H-bonding takes place between H and electronegative element present in the different molecules ofthe same substance (as in between H2 O and H2 O) or different substances (as in between H2 O and NH3 ) (A) Inwater molecules: Due to polarnature ofH2 O, thereis association ofwatermolecules giveing a liquid state ofabnormally highb.p.: H – 2 O .... H – 2 O .... H – 2 O .... H – 2 O | | | | δ H δ H δ H δ H Ifwe actuallyobserve, one water molecules is joined to four water molecules- two with H-atoms and other two withO-atom. Thus coordinationnumberofwater molecule inwater is four. CHEMISTRY Theory Notes on Chemical Bonding-VI
- (B) In HFthere is againlinear chainassociation byH-bonding However inthe gaseous state, severalpolymeric forms oftheHFmolecules exist inwhichthe monomers are held together throughH-bonding. It exist as (HF)6 (cyclic), (HF)5 (cyclic), (HF)2 (diner) upto and extent (HF, monomer) (C) Carboxylic acid dimerises is gaseous state due to H-bonding (D)Alcoholis said to behighlysoluble inwaterdue to crossed intermolecularH-bonding (betweenH2 O and R-OH molecules). R H R | | | .... δ O – δ H .... δ O – δ H .... δ O – δ H However isomeric ether is less soluble inwater due to its (ether) non-polar nature. CH3 –CH2 – O – H CH3 – O –CH3 polar less polar Intra-MolecularH-bonding- This type ofbonding results betweenhydrogen & an electronegativeelement both present inthe same molecule. This type ofbonding decreases the boiling point ofthe compd. (A) (B) (C)
- Nature and importance of H-Bond- (1) H-bond is merelyan alectrostatic force rather than a chemicalbond. (2) H-bond never involves more thantwo atoms. (3) Bond energyof H-bond is in the range of 3 to 10 k cal/mole. Whichis about (1/10)th the energyof a covalent bond (50 to 100 k cal/mole) (4)Withtheincrease ofelectronegativityanddecreaseinsizeoftheatomto whichhydrogeniscovalanthy linked. Thestrength ofthe H-bond increases as HF----H > H–O ----H > H–N ---- H 10 kcal/mol 7 kcal/mol 3 kcal/mol Some properties explained by H-bonding- (1) HighB.P. &M.P. ofNH3 , H2 O and HFincomparisonto hydrides ofother elements ofV,VI andVII grps. to which N, O and F belong respectively. (2) H2 O exists as liquid under normalconditions whereas H2 S exists as a gas. (3) Asubstance is said to be insoluble in water if it does not form H-bonding with H2 O. Therefore, Alkanes,Alkenes &Alkynes are in soluble whereas alcohols & acids are soluble in water. * Ether isalso insoluble. * Chlorine hasthesame electronegativityas Nitrogenbut does not formstrongH-bonding. This is because ofthelarger size thanthat ofnitrogen withthe result that its electrostatic attractionsare weak. (4) Dissociation- Inliquid solutions, HFdissociates& gives the difluoride ion(HF2 - ) insteadoffluoride ion (F- ). This is due to H-bonding in HF. This explains the existance ofKHF2 . Association- Due to H-bonding carboxylic acid exict as dimers due to intermolecular H-bonding as shown below. (5) Solubilityinwater: Compounds whichformhydrogenbonding withwater willdissolvein water. For example lower alcoholare soluble in water because their molecules can formH-bonds withwater .... H - O .... H - O .... H - O C2 H5 H C2 H5 Similarlyammonia and amines are soluble inwater due to H-bonding with water. (6) Thesubstances whichcangivemore extensive hydrogenbondingwillhave higher viscosityand high surface tension e.g. tension surface and viscosity Higher Glycerol Glycol Ethanol