Hybrid orbitals

Hybridization of orbitals (L. Pauling) the combination of two or more atomic orbitals forms a new set of atomic orbitals, called hybrid orbitals We deal with three types of hybrid orbitals sp3 (one s orbital + three p orbitals) sp2 (one s orbital + two p orbitals) sp (one s orbital + one p orbital) Overlap of hybrid orbitals can form two types of bonds depending on the geometry of overlap  bonds are formed by “direct” overlap  bonds are formed by “parallel” overlap

sp3 Hybrid Orbitals

– each sp3 hybrid orbital has two lobes of unequal size – the sign of the wave function is positive in one lobe, negative in the other, and zero at the nucleus – the four sp3 hybrid orbitals are directed toward the corners of a regular tetrahedron at angles of 109.5°

sp3 Hybrid Orbitals

– orbital overlap pictures of methane, ammonia, and water

Hydrocarbons

• Hydrocarbons are said to be unsaturated when they contain multiple bonds (i.e C-C double bonds or triple bonds). Alkenes or alkynes are therefore known as unsaturated hydrocarbons. Unsaturated compounds The double bond in alkenes, such as ethene (ethylene), and the triple bond in alkynes, such as ethyne (acetylene), are the result of the ability of the atomic orbitals of carbon to adopt sp2 and sp hybridization, respectively. Thus, the s bonds in ethene are derived entirely from carbon-based sp2 hybrid orbitals: Csp2 – Csp2 for the C – C bond, and Csp2 – H1s for holding the four hydrogens Ethene and Ethyne Sp2 Hybridization

• Carbon atom may not use all its s- orbital and p-orbital for hybridization, it may use one s- orbital and two p-orbital to form 3Sp2 hybrid orbitals • 2s+ 2px+ 2py sp2 Hybrid Orbitals

– the axes of the three sp2 hybrid orbitals lie in a plane and are directed toward the corners of an equilateral triangle – the unhybridized 2p orbital lies perpendicular to the plane of the three hybrid orbitals

BONDING IN ETHENE Bonding in Formaldehyde sp Hybrid Orbitals

– two lobes of unequal size at an angle of 180° – the unhybridized 2p orbitals are perpendicular to each other and to the line created by the axes of the two sp hybrid orbitals

Bonding in Acetylene, C2H2 Hybrid Orbitals

Groups Orbital Predicted Types of Bonded Hybrid- Bond Bonds to Carbon ization Angles to Carbon Example Name H H 4 sp3 109.5° 4 sigma bonds H-C-C-H Ethane H H

H H 2 3 sigma bonds 2 sp 120° Ethylene and 1 pi bond C C H H

2 sigma bonds 2 sp 180° H-C C-H Acetylene and 2 pi bonds Bond Lengths and Bond Strengths

Orbital Bond Length Bond Strength Name Formula Bond Overlap (pm) [kJ (kcal)/mol]

H H C-C sp3-sp 3 153.2 376 (90) Ethane H-C-C-H C-H sp3-1s 111.4 422 (101) H H

H H C-C sp2-sp2, 2p-2p 133.9 727 (174) Ethylene C C 2 H H C-H sp -1s 110.0 464(111)

C-C sp-sp, tw o 2p-2p 121.2 966 (231) Acetylene H-C C-H C-H sp-1s 109.0 556 (133)