Covalent Bonding What is covalent bonding Covalent Bonds overlap of orbitals bond bond Molecular Orbitals Hybrid Orbital Formation Shapes of Hybrid Orbitals Hybrid orbitals and Multiple Bonds resonance structures 1 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 110 2 Covalent Bonding H H H2 H 1s H 1s 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 3 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 for H2 4 Bond Types bond results from head on overlap of orbitals electron density is symmetric about the internuclear axis between nuclei Examples s s s p pp bond results from sideways overlap of orbitals electron density is above and below the internuclear axis Two p orbitals 5 Bonding in CH4 Carbon ground state 1s2 2s22p2 1s H 2p C Using only unpaired subshell electrons Expect CH2 molecule with 90o bond angles The molecule would not have an octet on carbon H C H 6 Orbital Hybridization 1 1s Promote electrons on C 2s 2p 1s 2s 2p Rearrange 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 shake well 1s 3 2s 2p 1s sp3 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 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 H C 7 H H 109 5 H Orbital Hybridization NOTE start with four atomic orbitals s px py pz end up with four hybrid orbitals 4 sp3 of hybrid orbitals sum of the superscripts of the named atomic orbital in hybridization Composition of hybrid orbitals 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 molecular geometry 8 sp3 Hybrid Orbitals Four atomic orbitals mix to form four hybrid orbitals 1xs 3xp 4 x sp3 9 sp and sp2 Hybrid Orbitals Two atomic orbitals mix to form two hybrid orbitals 1xs 1xp 2 x sp Three atomic orbitals mix to form three hybrid orbitals 1xs 2xp 3 x sp2 10 Summary Problem Can t 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 Use of electron domains or EPG to determine how many hybrid orbitals are needed add atomic orbitals to the hybridization in the order of s p p p d d Summary of hybridization types The hybridization scheme can be deduced from the electron pair geometry of the molecule Number of electron pairs Atomic orbitals used Hybrid type formed Electron pair geometry Examples 2 s p two sp linear 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 PF5 SF4 bipyramidal BrF3 6 s p p p d d six sp3d2 octahedral BeF2 HgCl2 SF6 ClF5 XeF4 PF6 12 Multiple Bonds ethylene shape about C trigonal planar hybrid orbitals on C sp2 H H C C H H bond angles approximately 120 One s and two p atomic orbitals combine to form 3 sp2 hybrid orbitals One C C and two C H bonds on each carbon are formed using sp2 orbitals on carbons bonds 13 Multiple bonds These p orbitals can overlap sideways bond Double bond 1 bond 1 bond Triple bond 1 bond 2 bond Ethene ethylene 14 Ethyne acetylene Orbital Theory of Bonding explains Why rotation about double bond does not occur For this to happen a bond must be broken Requires large energy 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 better overlap stronger bond 15 Delocalized Orbitals e pair geometry trigonal planar hybrid orbitals on N and O are sp2 N and O have singly occupied p orbitals Difference between localized and delocalized bonding Delocalized bonding brings added stability to a molecule Molecules with resonance structures have delocalized bonding Extra stability less reactive 16 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 17 Classes of Organic Compounds Classification is necessary to manage the large number of compounds HYDROCARBONS Simplest organic compounds Contain only C and H Classes of Hydrocarbons 1 Alkanes C C all single bonds 2 Alkenes one or more double bonds 3 alkynes C C one or more triple bonds C 4 aromatic C alternating single double bonds in a ring resonance extra stability H low reactivity Example benzene ring H H H H H 18 ALKANES Name Molecular formula Condensed molecular formula methane CH4 ethane C 2H 6 CH3CH3 propane C 3H 8 CH3CH2CH3 butane C4H10 CH3CH2CH2CH3 pentane C5H12 CH3CH2CH2CH2CH3 Greek Latin numerical prefix after 5 carbons pent hex hept oct non dec In general CnH2n 2 Lewis Structures First draw the