Hydrogen Bond PDF

# Hydrogen bond - In compounds of hydrogen with strongly electronegative elements, such as fluorine, oxygen and nitrogen, the electron pair shared between two atoms lies far away from hydrogen atom. - This results into hydrogen atom becomes electropositive with respect to other atom. - This phenome...

# Hydrogen bond - In compounds of hydrogen with strongly electronegative elements, such as fluorine, oxygen and nitrogen, the electron pair shared between two atoms lies far away from hydrogen atom. - This results into hydrogen atom becomes electropositive with respect to other atom. - This phenomenon of charge separation in Hydrogen fluoride is represented as H+δ - -F-δ. - Such a molecule is said to be polar. - The molecule behaves as a dipole because one end carries a positive charge and other end a negative charge. - The electrostatic force of attraction between such molecules should be strong. - Because positive end of one molecule attracts and is attracted by the negative end of other molecule. - Thus, two or more molecules may associate together to form large clusters of molecules. # Nature of Hydrogen bond - The hydrogen bond is a class in itself. - It arises from electrostatic forces between the positive end (pole) of one molecule and the negative end (pole) of another molecule of the same or some other polar substance. - E.g. Extensive hydrogen bonding occurs between molecules of water, molecules of ethanol and also between molecules of water and molecules of ethanol. - The strength of hydrogen bond is found to vary between 10-40 KJ per mole (i.e.per 6.022 x 10^23 bonds.) - That of covalent bond has found to be of the order of 400KJ mole. - Thus, a hydrogen bond is very much weaker than a covalent bond. # Hydrogen Bonding - Essential requirements for formation of hydrogen bond: - A hydrogen atom must be directly bonded to a highly electronegative atom (e.g. F, O and N) - An unbonded pair of electrons (lone pair electrons) is presented on the electronegative atom. # Types of Hydrogen bonding - The covalent boding between a hydrogen atom and a strongly electronegative atom becomes 'polar'-covalent. - The 'charged hydrogen 'ion' can be attracted to a electronegative atom, such as nitrogen, oxygen or fluorine. - Hydrogen bond should not be confused with a covalent bond to hydrogen. - Types of hydrogen bonds: - Intermolecular (between molecules) - Intramolecular (within a molecule) - E.g. of hydrogen bonding: water (responsible for the high boiling point of water compared to say H2S), DNA, partly responsible for the secondary, tertiary, and quaternary structures of proteins and nucleic acids, Polymers # TYPES OF HYDROGEN BONDING - Intermolecular hydrogen bond: - Hydrogen bond formed between two molecules - Intramolecular hydrogen bond: - Hydrogen bond formed between two different atoms in the same molecule # A hydrogen bond - 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, oxygen or 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. # These bonds - 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-Keesom force, 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 group 16 hydrides. - Intramolecular hydrogen bonding is partly responsible for the secondary, tertiary and quaternary structures of proteins and nucleic acids. # Properties of Hydrogen Bond - 1) It is a bond between two electronegative atoms only. It never involves more than two atoms (excluding H atoms). - 2) Bond Energy of a H-bond is in the range of 3-10 Kcal/mole. While that of a normal covalent bond is in the range of 50-100 Kcal/mole. Thus a H-bond(H---B) is much weaker than a cobalent bond A-H. - The difference in energy between A-H and H...B bond indicates they have different bond length which in turn shows that H atom in A-H...B is never midway between two atoms A and B. - It is rather always nearer to atom A which is covalently bonded to H-atom. - H-Bond has more energy(=3-10 Kcal/mole) than Vander Walls forces(= 1 Kcal/mole). - 3) The formation of H-bond does not involve any sharing of electron pairs. It is therefore quite different from a covalent bond. # Properties of Hydrogen Bond - 4) H-bond in A-H....B is formed easily when both the atoms A & B are highly electronegative. - Thus the ease of formation of H-bond in A-H...B increses with the increases in the electronegativity value of atom A indicating as ΧA. - This tendency clearly explains that tendency of A-H bond to form a H-bond increases from N-H through O-H to F-H as ΧN < ΧO < ΧF - This tendency decreases in passing from O-H to S-H or from F-H to Cl-H because ΧO>ΧS and ΧF> ΧCI - This shows that F atom with highest electro negativity(ΧF) forms the stronger H-bond. # Formation of hydrogen bonds between H₂O molecules. - Electrostatic attraction exists between partial positive charge of H atom and the lone pair electrons of O atom of another H₂O. # Formation of hydrogen bonds between NH₃ molecules - Electrostatic attraction exists between partial positive charge of H atom and the lone pair electrons of N atom of another NH₃. # Consequences of Hydrogen Bonding: - 1. State of H₂O (liquid) and H₂S (gas). - The ease of formation of H-Bond in A-H... B decreases with the decrease in the electronegatively of atom B. - Since ΧO > ΧS there is a considerable hydrogen bonding in H₂O while in H₂S, it is absent. In other words, H₂O molecules can associate to-gether to form a polymerixed molecule. (H₂O)n in which hydrogen atom acts as a bridge between two oxygen atoms which are highly electronegative. Due to formation of this polymerized (H₂O)n molecule containing Hydrogen bonds water exists as a liquid. - In H₂S there is no Hydrogen bond formation. Hence, it does not form cluster. Hence, it exists as a gas. # Ice has less Density than water. - In the crystal structure of ice, the oxygen atom is surrounded by four H-atoms. - Two H-atoms are linked to O-atom by covalent bonds and the remaining two H-atoms are linked to O-atom by two H-bonds shown by dotted line. - Thus, in ice, every water molecule is associated with four other water molecules by H-bonding in a tetrahedral fashion. - Ice has open structure with a large empty space due to existence of H-bonds. - As ice melts at 0°C. number of H-bonds are broken down and the space between water molecules decreases so that water molecules move close together. - The density of water therefore increases from 0°C to 4°C at which it is maximum.