Test 1: Alkanes through Conformations

Does not include Functional Groups or Alkane Nomenclature

Hydrocarbons

Compounds containing only hydrogen and carbon (alkanes, alkenes, and alkynes)

Which of these reacts with hydrogen: Alkane, alkene, alkyne?

Alkene and Alkyne (hydrogenation, forms alkanes)

Constitutional or Structural Isomers

compounds with the same molecular formula that differ in the order in which the atoms are bonded to one another

Saturated Hydrocarbons

Alkanes (only carbon-carbon single bonds)

Unsaturated Hydrocarbons

Alkenes, Alkynes, Arenes

Arenes

Hydrocarbons with one or more benzene-like rings

Boiling point, melting point, and density increase with ______ # carbons

increasing

(BP increases by 30°C with each additional CH2)

Alkanes with 1-4 carbons are

gases

Alkanes with 5-17 carbons are

liquids

Alkanes with >17 carbons are

solids

Alkanes are not reactive (true/false)

true

Halogenation

Alkane + Halogen → alkyl halide + H(halogen) + other products (catalyzed by light)

Oxidation of Alkanes

A reaction that either removes a hydrogen atom from a carbon or adds an electronegative element to the molecule (O, N, S, or a Halogen) → includes combustion reactions

Combustion of Alkanes

Alkane + Oxygen → CO2 + H2O

(catalyzed by a spark)

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Complete Combustion:

* yields CO2 and H2O

Incomplete Combustion:

* yields CO or just C and H2O

Heat of Combustion

Energy released when a compound is completely oxidized to CO2 and water

(depends mostly on the number of CH2 units: approximately 157 kcal/ methylene unit)

Explain ‘Like Dissolves Like’

A Solute will only dissolve in a solvent if both agree in terms of being Polar, Protic, or Donor molecules

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Polar: has a dielectric constant > 15

Protic: can H-Bond (if there are H on O, N, F)

Donor: can donate an e pair from O or N

Acidity Trends

1. (Charge Effect) positively charged atoms are more acidic
2. (Element Effect) acidity increases with elements going across and down the PT
3. (Hybridization Effect) acidity is greatest in molecules with high s-character (sp3< sp2 < sp)
4. (Resonance Effect) stronger acids have conjugate bases with charges delocalized by resonance
5. (Polar effect) Strong acids have a higher amount of electronegative atoms closer to the acidic oxygen (goes with charge effect)

Reduction of Alkenes and Alkynes

a reaction that either adds H atoms or removes an electronegative atom from the molecule

Steric Energy

(in an isolated molecule in gas phase at 0° K)

Relative energy of a conformation or stereoisomer calculated using classical mechanics

Stretch

(bond length)

Energy associated with stretching or compressing bonds from their optimal length

Bend

(bond angle)

Energy associated with deforming bond angles from their optimal angle

Stretch-Bend

Energy required to stretch two bonds involved in a severely compressed bond angle

Dipole-Dipole

Energy associated with the interaction of bond dipoles

Out of Plane

Energy required to distort a trigonal center out of planarity

Torsional Strain

Destabilization from eclipsing bonds on adjacent atoms

Van Der Waals Strain

Destabilization from two atoms being too close together

Dimensional Model

Newman Projection Model

Graphical Model of ΔE when a molecule rotates about a single bond (ethane, in this example)

“E” is when the molecule is Eclipsed (highest energy)

“S” is when the molecule is Staggered (lowest energy)

Eclipsed

Gauche

Anti

Substituted Ethanes

* an exception to the lowest energy conformation rule
* sometimes, a **gauche** conformation is preferred over staggered **if X and Y are electronegative substituents**

Rotational Barriers

* increases with the number of CH3/H eclipsing interactions
* (with an electronegative middle atom, or a lone pair) the rotational barrier increases with the number of H/H eclipsing interactions (due to lone pair bubble colliding with them)

Relative Energies of Molecular Conformations

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Cyclopropane

* Planar
* Steric Energy: \~128 kj/mol
* Bond angles: 60°
* Tetrahedral (sp3 bond angles): 109.5°
* Maximum overlap cannot be achieved
* Angle, Torsion, Van Der Waals
* Eclipsed (in photo)

Cyclobutane

* Puckered
* Steric Energy: 122 kj/mol
* Angle (C-C-C: 88°), Torsion, Van Der Waals
* Partially Eclipsed (in photo)

Cyclopentane

* Envelope
* steric energy: 48 kj/mols
* Torsion, Van Der Waals
* Partially Eclipsed (in photo)

Puckering

allows bond angles to be at or close to the tetrahedral angle (109.5°) and minimizes torsional strain (electron-electron repulsions in eclipsed bonds) between adjacent C-H bonds

Chair Conformations

Equatorial Bonds

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Axial Bonds

Half Chair

Boat

Relative Stability of Chair, Half-Chair, and Boat Conformations

High Energy

* half-chair
* boat
* twist boat
* chair

Low Energy

Half chair conformation

Twist Boat Conformation

Substituents on cyclohexanes prefer to occupy _____ positions

Equatorial positions due to 1,3 diaxial interactions

Compound Newman Projection Models: Anti

Compound Newman Projection Models: Gauche

Relationship of Equatorial/axial percentage of possibility positions to K

K = \[equatorial %\]/\[axial %\]

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(ex in photo) K = 95/5 = 19

Gibbs Free Energy

ΔG = -RT(lnK)

Disubstituted Cyclohexanes

two substituents on a cyclohexane

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Cis:

* substituents on the same side of the ring

Trans:

* substituents on opposite sides of the ring

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Determine cis/trans stability by calculating ΔG°

Bicyclic Compounds

* Conformationally locked (chairs cannot flip back and forth between conformations)

Rank the stability of these Newman Projections

III < IV < I < II

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* Eclipsed conformers are less stable than staggered conformers due to torsional strain
* III < IV because there is more steric strain due to eclipsing methyl groups
* I < II because of the higher steric strain of the gauche methyl groups

Drawing Alternate Cyclohexane structures

* chair inverts (over x axis)
* axial substituents become equatorial
* equatorial substituents become axial
* all substituents will maintain the direction they are facing (up stays up, down stays down)

Calculate the difference in ∆G due to 1,3 diaxial strain between these two structures given A-values

1\.0 kcal/mol

Which 2D figure represent the chair structure?

I

Which conformer is more stable?

Conformer 1

Bronsted-Lowry Acid

Proton Donor

Bronsted-Lowry Base

Proton Acceptor

Strong Acid

completely ionizes in water (have weak conj. bases)

Weak Acid

only partially dissociated in water (have strong conj. bases)

As acid strength increases, the basicity of the conj. base _______

decreases

Low pKa means

high acidity, the better ability of an atom to stabilize a negative charge

Acidity Constant (Ka)

\[A-\]\[H+\]/\[AH\]

the greater Ka, the more acidic

pKa

\-log\[Ka\]

Equilibrium favors the side with the ________ acid

weaker

Lewis Acid

electron pair acceptor (any species with an electron-deficient atom)

Lewis Base

electron pair donor (any species with an unshared pair of electrons)

Nucleophile

electron-rich atom, often negatively charged, with a free lone pair to donate to another atom

Electrophile

electron-poor atom with a low-lying vacant or easily vacated orbital; wants to accept electrons from a nucleophile

Relative importance of resonance structures

Most Important


1. all octets are filled
2. negative charges exist on the most electronegative atoms
3. charge separation is minimized

Atomic Orbitals

unhybridized orbitals on an atom (s, p, d)

Linear combination of atomic orbitals

individual wave functions (orbitals) combine to form hybrid atomic orbitals (sp, sp2, sp3) and molecular orbitals (*σ, σ*, π, π*)*

Hybrid Atomic Orbitals

Combination of atomic orbitals from the __**same**__ atom

Molecular orbital

combination of atomic orbitals from __**different**__ atoms

Conservation of Orbitals

when you add orbitals together, you always end up with the same amount of orbitals that you started with

bonding

(+/+ or -/-) electron density centered between nuclei

anti-bonding

(+/-) generally has a node between nuclei

node

an area of zero electron density

In stable bonding situations, usually only the ______ and _________ orbitals are occupied

sigma, pi

sigma bonding orbitals

* cylindrically symmetrical molecular orbitals
* electron density is centered along the axis of the bond
* single bonds = sigma bonds

pi bonding orbitals

* not cylindrically symmetrical
* electron density is located above and below the axis of the bond
* double and triple bonds = pi bonds

Structure

determining the way in which atoms are put together in space to form complex molecules

Mechanism

understanding the reactivity of molecules: how and why chemical reactions take place

Synthesis

building complex molecules from simple using chemical reactions

Bond lengths

dependent on atomic size, bond order, and hybridization

Multiple Bonding

bond length is determined by bond order (Single > Double > Triple)

Effect of Hybridization on Length of Single Bonds

C-H and C-C bonds shorten slightly with increased S character on Carbon (sp > sp2 > sp3)

Bond dissociation energies

* energy for homolytic bond cleavage to uncharged radical fragments
* dependent on the specific molecular structure

Bond Strengths

* bond energies for a certain bond averaged over many different molecules
* for multiple bonds: strength is for single < double < triple bonds