1 Covalent Bonding What is covalent bonding? Covalent Bonds: overlap of orbitals σ-bond π-bond Molecular Orbitals Hybrid Orbital Formation Shapes of Hybrid Orbitals Hybrid orbitals and Multiple Bonds resonance structures2 Molecular Orbitals • Lewis structures: accounting for bonding and lone-pair electrons (where are the electrons?) • VSEPR: Electron-pair structure, spatial distribution of electrons (3D) How are bonds made? We know electron distribution in atoms: atomic orbitals: (s, p, d …) What is the electron distribution in molecules? Two models: Valence Bond Theory Valence orbitals on one atom overlap with valence orbitals on another atom: this overlap of orbitals is a covalent bond. Molecular Orbital Theory Not covered in Chem 1103 Covalent Bonding H + H → H2 H2 molecule Covalent bonding: overlap or combining of (two singly occupied) atomic orbitals to form a new (doubly occupied molecular) orbital, allowing for sharing of electrons by the two nuclei. H (1s) H (1s)4 H + H → H2 Two forces operating: • increased overlap of atomic orbitals (better sharing) brings atoms together • closer distance between nuclei increases positive-positive charge repulsion balance of forces a bond length (0.74 Å for H2)Bond Types σ-bond • results from head-on overlap of orbitals • electron density is symmetric about the internuclear axis: between nuclei Examples: π-bond • results from sideways overlap of orbitals • electron density is above and below the internuclear axis s-s s-p pp Two p-orbitals 56 Bonding in CH4 Using only unpaired subshell electrons: Expect: "CH2" molecule with 90o bond angles. The molecule would not have an octet on carbon. Carbon ground-state: (1s2)2s22p2 1s (H) 2p (C) HC HOrbital Hybridization 1. Promote electrons on C 1s 2s 2p1s 2s 2p1s1s 2s 2pshake wellsp3Rearrange valence electrons to yield half-filled orbitals Requires energy 2. hybridization Four atomic orbitals (2s + 3 × 2p) mix to form four hybrid orbitals (4 × sp3) 3. Bond formation: form a shared electron pair by combining 2 half-filled orbitals Form 4 C⎯H bonds by overlapping each hybrid sp3 orbital with an 1s orbital of hydrogen. σ-bond formation The new bonds are 109o apart. One of the four bonds formed by overlap of an sp3 orbital with a hydrogen 1s orbital CHHHH109.5°CHHHH109.5°78 Orbital Hybridization NOTE: start with four atomic orbitals s px py pz end up with four hybrid orbitals 4 sp3 The notation means that each hybrid is composed of 1/4 s and 3/4 p orbitals. Hybrid orbitals: combinations of atomic orbitals (on one atom). better for bonding (more directed) è molecular geometry # of hybrid orbitals (= sum of the superscripts) Composition of hybrid orbitals # of the named atomic orbital in hybridization9 sp3 Hybrid Orbitals Four atomic orbitals mix to form four hybrid orbitals 1 x s + 3 x p 4 x sp310 sp and sp2 Hybrid Orbitals Two atomic orbitals mix to form two hybrid orbitals 1 x s + 1 x p 2 x sp Three atomic orbitals mix to form three hybrid orbitals 1 x s + 2 x p 3 x sp2Summary Problem: Canʼt use atomic orbitals to describe bonding in molecules Solution: make molecular orbitals by mixing atomic orbitals (call them hybrid orbitals) Two atomic orbitals mix to form two hybrid orbitals Three atomic orbitals mix to form three hybrid orbitals Four atomic orbitals mix to form four hybrid orbitals Five atomic orbitals mix to form five hybrid orbitals Six atomic orbitals mix to form six hybrid orbitals Each hybrid orbital can accommodate 1 pair of electrons. Use VSEPR to determine shape of hybrid orbitals: the electron pairs will get as far from each other as possible. è Use # of electron domains (or EPG) to determine how many hybrid orbitals are needed è add atomic orbitals to the hybridization in the order of s, p, p, p, d, d12 Summary of hybridization types Number of electron pairs Atomic orbitals used Hybrid type formed Electron-pair geometry Examples 2 s, p two sp linear BeF2, HgCl2 3 s, p, p three sp2 trigonal planar BF3, SO3, CO32- 4 s, p, p, p four sp3 tetrahedral CH4, NH3, H2O, NH4+ 5 s, p, p, p, d five sp3d trigonal bipyramidal PF5, SF4, BrF3 6 s, p, p, p, d, d six sp3d2 octahedral SF6, ClF5, XeF4, PF6- The hybridization scheme can be deduced from the electron-pair geometry of the molecule.13 Multiple Bonds • ethylene: shape about C: trigonal planar hybrid orbitals on C sp2 bond angles approximately 120° C CHHHHOne C⎯C and two C⎯H bonds (on each carbon) are formed using sp2 orbitals on carbons. ( σ- bonds) One s and two p atomic orbitals combine to form 3 sp2 hybrid orbitals.Multiple bonds These p-orbitals can overlap, sideways: π-bond Double bond = 1 σ bond + 1 π bond Triple bond = 1 σ bond + 2 π bond Ethene (ethylene) Ethyne (acetylene) 1415 Orbital Theory of Bonding explains: • Why rotation about double bond does not occur For this to happen, a π bond must be broken è Requires large energy è Cannot be observed at room temperature Why double bonds occur frequently with C, N, and O but not with larger molecules Closer distance between p orbitals è better overlap è stronger bond16 Delocalized Orbitals N and O have singly occupied p-orbitals Difference between localized and delocalized π bonding Molecules with resonance structures have delocalized π bonding è Extra stability (less reactive) e- pair geometry : trigonal planar hybrid orbitals on N and O are sp2 Delocalized bonding brings added stability to a molecule.Organic Chemistry The chemistry of compounds containing C bonded to H. Often contains O, N, S, and halogens, also Millions of organic compounds are known. They are the main constituents of living matter: DNA, RNA and proteins Reasons for large numbers of organic compounds: 1. Carbon atoms can form strong, stable bonds to other C atoms (thus forming rings, chains, etc.) and to atoms such as H, O, N, S, halogens. (small size) 2. Carbon atoms form up to 4 bonds simultaneously: (valence of 4) molecules can be branched. 3. Carbon atoms form multiple bonds with C or with O, N, S: further structural variations are possible. (Small size and valence of 4) 17Classes of Organic Compounds Classification is necessary to manage the large number of