Bonding. Conceptually easier to view bonding if one considers electrons as waves. Consider a two dimensional standing wave

Transcription

1 Bonding Conceptually easier to view bonding if one considers electrons as waves Consider a two dimensional standing wave

2 If we constrain another point in the middle of the string we obtain the first harmonic The signs used (+ and -) are arbitrary Important point is the change in sign (called a node)

3 Wave function of s atomic orbital The region of space where electrons are held are called orbitals The property and shape of orbitals is dependent on what shell the electrons are held With 2 nd row atoms, the atomic orbitals can be either s or p (s orbitals are lower in energy than p orbitals) A wave function is simply a mathematical formula that describes the region of space the electron may reside in a given atomic orbital Atomic s orbital: In two-dimensions in three-dimensions No node therefore same sign throughout sphere Electron density dimishes further from the nucleus

4 Wave function of atomic p orbital Atomic p orbital: In two-dimensions in three-dimensions electron density - + distance nucleus Important points: Density is zero at nucleus Lobes on opposite sides of the nucleus are out of phase (different sign)

5 Combination of Two Waves When a bond forms it is a result of combination of two atomic wave functions

6 Combination of Atomic Orbitals Forms Molecular Orbitals

7 Energy Gain in a Covalent Bond As atomic orbitals combine they form a new molecular orbitals that allow electrons to be shared between the two nuclei

8 Bond Formation Bonds are formed by a combination of orbitals Orbitals location of electrons (on time average) (described by the wave function) phase describes the sign of an orbital Only orbitals of like phase can combine in a constructive manner to form a bonding region where the electrons can reside This bond lowers the energy of the structure as the electrons shield the two positively charged nuclei Always get the same number of molecular orbitals as atomic orbitals used Organic compounds are described in terms of the type of bonds (σ sigma and π pi bonds)

9 σ Bonds Electron density is symmetric along the internuclear axis

10 When combine two atomic orbitals we need to obtain two molecular orbitals In addition to bonding molecular orbital shown, also obtain an antibonding orbital Antibonding orbital is obtained by subtracting the two wave functions

11 In addition to combining s atomic orbitals, bonds can be formed by combining p atomic orbitals There are 3 atomic p orbitals - Each atomic p orbital is perpendicular (orthogonal) to the other two This geometry allows the p orbital to form a different type of bond

12 P orbitals can also form σ bonds Can add and substract two atomic p orbitals to obtain a bonding and antibonding σ bond Bonding molecular orbital (σ bond) Antibonding molecular orbital

13 If add a different p orbital a different type of bond can form Consider adding two atomic 2p y orbitals p bond Electron density is not symmetric about the internuclear axis s orbitals can only form σ bonds p orbitals can form either σ or π bonds

14 Three Views of Bonding for Organic Compounds 1) Total electron density (nature) 2) Molecular Orbitals (MOs) (computer) 3) Bonds from hybrid atomic orbitals (AOs) (students) Conceptually very useful concept to explain structure and reactivity

15 Hybrid Orbitals If bonds formed only from atomic s or p orbitals then the bond angles would always be 90 Also could not fill atoms valence shell with covalent bonds Consider carbon In a covalent bond electrons are shared between two atoms (each atom donates one to the sharing) In carbon there are only two unpaired valence electrons therefore only two covalent bonds are possible

16 An atom can combine its atomic orbitals to form hybridized orbitals Same rules apply for combining atomic orbitals to form hybrid orbitals 1) Get same number of hybridized orbitals as starting atomic orbitals used to form hybrid 2) Shape of hybridized orbitals is obtained by the mathematical addition of the wave functions for the atomic orbitals The name (designation) of hybridized orbitals merely refers to the number and type of atomic orbitals used in the formation

17 sp Orbital Combine one s orbital with one p orbital If the orbitals are substracted then an indentical hybridized orbital is obtained directed 180 from the first

18 Realize that if one s orbital and one p orbital are hybridized then two p orbitals remain Consider acetylene (also called ethyne) - When looking at a Lewis dot multiple bond structure, only one σ bond can be between two atoms - Therefore all additional bonds are always π bonds

19 Hybridized Bonding View of Acetylene The two hybridized sp orbitals form σ bonds with the other carbon and a hydrogen, The remaining p orbitals form two additional π bonds to form a total of three bonds between carbon

20 sp 2 Hybridization - Can also hybridize by combining one s orbital with two p orbitals (would allow formation of three covalent bonds one from each sp 2 hybridized) All three sp 2 orbitals are in the same plane (120 apart from one another)

21 The remaining p orbital is perpendicular to this plane The hybridized sp 2 orbitals can form σ bonds The p z orbital can form a π bond

22 sp 3 Hybridization To form four equivalent bonds carbon can hybridize all of its valence orbitals (three p and one s to form four sp 3 hybrids) The four sp 3 hybridized orbitals have a bond angle of 109.5

23 How to View 3-Dimensional Objects Organic chemists use a wedge and dash line system to designate stereochemistry Wedge line object is pointing out of the plane Dash line object is pointing into the plane H H H H

24 Bond Rotation Bonding between two atoms is a result of the overlap of the orbitals from each atom If only a σ bond is formed between two atoms, then the bond can be rotated a full 360 and maintain the same overlap of the orbitals - Consequence of orbitals being symmetric about the internuclear axis Causes substituents to have different relative placement depending upon bond rotation

25 Rigidity of π Bonds In contrast, π bonds are not symmetric about the internuclear axis If π bonds would rotate the bond would lose overlap and hence the bond would be broken

26 Cis versus Trans The rigidity of a π bond causes different compounds to be formed that cannot rotate to interconvert (hence they have different physical properties) If substituents are on the same side of the double bond, it is called a cis compound If substituents are on opposite sides of the double bond, it is called a trans compound

27 Isomers The cis and trans compounds are one type of relationship that are called isomers Isomers are any two compounds that are different but have the same molecular formula Isomers can be further classified as being either stereoisomers (having same formula but differing in the three dimensional arrangement, thus configurational) or constitutional isomers (compounds are bonded in a different pattern)

28 In summary: s orbitals can only form σ bonds p orbitals can form either σ or π bonds Hybridized orbitals form σ bonds Multiple bonds are formed with one σ bond and additional π bonds lone pair of electrons often go into hybridized orbitals Why do atomic orbitals hybridize? Allows some atoms to form more covalent bonds Allows atoms to form bonds at different angles Molecular geometry is therefore dependent upon the type of hybridization undertaken

29 Hybridization Hybridization affects geometry Likewise geometry affects hybridization Key points only electrons in p orbitals can resonate (need orbital overlap) Resonates sp 2 hybridization Cannot resonate sp 3 hybridization

30 How to determine molecular structure Determine number of σ bonds from an atom Determine number of lone pair of electrons on an atom that are not involved in resonance The addition of the above two numbers equals the number of hybridized orbitals required Remaining atomic p orbitals are used to form multiple bonds If a lone pair is involved in resonance it must reside in an atomic p orbital to allow orbital overlap

31 Experimental Evidence for Resonance Affecting Hybridization and Geometry Consider an amide group: If the lone pair is resonating with the carbonyl then the N atom is sp 2 hybridized, If the lone pair is not resonating then the N atom is sp 3 hybridized Evidence that the N is resonating and is sp 2 hybridized: Infrared (IR) spectroscopy (chapter 12) can measure energy required to cause a bond vibration, energy of carbonyl stretch is low compared to other carbonyls not in resonance Nuclear Magnetic Resonance (NMR chapter 13) can measure electronic environment around a nucleus, observe two distinct environments for CH 3 groups thus cannot rotate freely X-ray can measure atomic positions, amide structures display nearly 120 bond angles for C-N-C bond angles while this angle is close to for C-N-C bond angle in amines Acidity can measure how easy it is to protonate a nitrogen in an amide versus a nitrogen in a nonresonating amine, much harder to protonate amide lone pair due to resonance