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CHEM 109A Organic Chemistry

Questions? (non chemistry) Chika Anyiwo, undergraduate advisor

anyiwo@chem.ucsb.edu

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CHEM 109A Organic Chemistry

Questions? (chemistry!) CLAS tutor for this 109A course

Timothy Ngo: txngo@umail.ucsb.edu Sign up!

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https : / / labs .chem.ucsb .edu/zakar ian /armen/courses .html

CHEM 109A Organic Chemistry

Chapter 1 Electronic Structure and bonding

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Ch. 1: Important topics/concepts

Structures: Lewis structures (Kekule, condensed, skeletal) properly placing lone pairs of e, formal charges

Bonds: σ and π bonds (single bonds, double bonds, triple bonds) σ formed by orbital overlap, s+s, s+p, p+s, or hybridized π formed by overlap between p orbitals double bond = 1 σ bond + 1 π bond triple bond = 1 σ bond + 2 π bonds

O BF3H

HO B

H

H F

FF=

condensed skeletal Lewis

?= Lone pairs? Formal charges?

Covalent Bonds: polar, or nonpolar (electronegativity) compare polarity of bonds, such as H-C vs H-N vs H-O vs H-F

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Ch. 1: Important topics/concepts

Hybrid orbitals: s and p orbitals mix to form new hybrid orbitals (comes from valence-shell electron-pair repulsion (VSEPR) theory explains geometry of CH4 and many other molecules

Geometry (shape of molecules):

CH4: tetrahedral, four sp3 orbitals, bond angle 109o, H2C=CH2, ethylene, three sp2 orbitals, one p orbital, bond angle ~120o

HC CH, acetylene, two sp bonds, 2 p orbitals, bond angle 180o

Bond length and strength: shorter bonds are stronger.

ETHYLENE ACETYLENE

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The Distribution of Electrons in an Atom

•  Thefirstshellisclosesttothenucleus.•  Theclosertheatomicorbitalistothenucleus,theloweritsenergy.•  Withinashell,slowerinenergythanp.

n

l

ml

2 e per orbital

quantumnumbers

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The Distribution of Electrons in an Atom

•  Thefirstshellisclosesttothenucleus.•  Theclosertheatomicorbitalistothenucleus,theloweritsenergy.•  Withinashell,slowerinenergythanp.

n

l

ml

2 e per orbital

lobes with opposite phases 2s orbital, presents

only one phase for overlap

x

y z

quantumnumbers

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Achieving a Filled Outer Shell by Sharing Electrons

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How Many Bonds Does an Atom Form?

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Nonpolar and Polar Covalent Bonds

Nonpolarcovalentbond=bondedatomsarethesameorhavesimilarelectronegativities.

Polarcovalentbond=bondedatomshavedifferentelectronegativities.

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Nonpolar and Polar Covalent Bonds Nonpolarcovalentbond=bondedatomsarethesameorhavesimilarelectronegativities.

Polarcovalentbond=bondedatomshavedifferentelectronegativities.

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Nonpolar and Polar Covalent Bonds Nonpolarcovalentbond=bondedatomsarethesameorhavesimilarelectronegativities.

Polarcovalentbond=bondedatomshavedifferentelectronegativities.

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ELECTRONEGATIVITY OF ELEMENTS

Differences in electronegativity between atoms lead to polar bonds The bigger the difference, the more polar the bond

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The Greater the Difference in Electronegativity, the More Polar the Bond

Nonpolarcovalentbond:electonegativitydifference<0.5

Polarcovalentbond:electonegativitydifference0.5–1.9

Electronegativitydifference>1.9: electronsarenotshared;atomsareheldtogetherbytheattractionofoppositecharges

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Dipole Moment

Thegreaterthedifferenceinelectronegativity,thegreaterthedipolemomentandthemorepolarthebond.

Dipolemoment=sizeofthechargexthedistancebetweenthecharges

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Dipole Moment

Thegreaterthedifferenceinelectronegativity,thegreaterthedipolemomentandthemorepolarthebond.

Dipolemoment=sizeofthechargexthedistancebetweenthecharges

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Electrostatic Potential Maps

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Lewis Structures Important items: lone pairs, formal charges

O BF3H

HO B

H

H F

FF=

condensed skeletal Lewis

?=

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Carbon Forms Four Bonds

Ifcarbondoesnotformfourbonds,

ithasacharge(oritisaradical).

C

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Nitrogen Forms Three Bonds

Nitrogenhasonelonepair.

Ifnitrogendoesnotformthreebonds,itischarged.

N

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Oxygen Forms Two Bonds

Oxygenhastwolonepairs.

Ifoxygendoesnotformtwobonds,itischarged.

O

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Hydrogen and the Halogens Form One Bond

Ahalogenhasthreelonepairs.Ifhydrogenorhalogendoesnotformonebond,ithasacharge(oritisaradical).

Cl

H

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The Number of Bonds Plus the Number of Lone Pairs Equals Four

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Lewis Structures

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Formal Charge

FormalCharge=

the#ofvalenceelectrons–

(the#oflone-pairelectrons+the#ofbonds)

Important items: lone pairs, formal charges

O BF3H

HO B

H

H F

FF=

condensed skeletal Lewis

?=

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How to Draw a Lewis Structure

NO3–(anionofnitricacidHNO3)

Determinethetotalnumberofvalenceelectrons(5+6+6+6=23).Becausetheyarenegativelycharged,addanotherelectron=24.AvoidO—Obonds.

Checkforformalcharges.

review Gen. Chem. and practice!

Homework: problems 49, 54, 55

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Kekulé Structures and Condensed Structures

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Kekulé Structures and Condensed Structures

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Kekulé Structures and Condensed Structures

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Skeletal Structures

Skeletalstructuresshowthecarbon-carbonbondsaslines,butdonotshowthecarbonsorthehydrogensthatarebondedtothecarbons.

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Back to Bonding: Forming a Sigma Bond

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Waves Can Reinforce Each Other Waves Can Cancel Each Other

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Atomic Orbitals Combine to Form Molecular Orbitals

OrbitalsareConserved#ofMolecularOrbitals=#ofAtomicOrbitalsCombined

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Side-to-Side Overlap of In-Phase p Orbitals Forms a π Bond