Chem 41A - Midterm 2

Joe O'Connor

What is this functional group

Alkane

What is this functional group

Haloalkane

What is this functional group

Alcohol

What is this functional group

Ethers

What is this functional group

Thiols

What is this functional group

Alkenes

What is this functional group

Alkynes

What is this functional group

Aromatic Compounds

What is this functional group

Aldehydes

What is this functional group

Ketone

What is this functional group

Carboxylic Acid

What is this functional group

Anhydrides

What is this functional group

Esters

What is this functional group

Amides

What is this functional group

Nitriles

What is this functional group

Amines

What is the valence electrons in order from Boron to Fluorine. What about the amount of bonds they like

Valance electrons: 3, 4, 5, 6, 7

Amount of bonds they line: 3, 4, 3, 2, 1

How should the wedge be placed when with a dash.

The dash should always be above the wedge

As you increase in orbitals is there an increase or decrease in energy

There is an increase in energy

What is the difference between a trans-, cis-, and

What is a simple way to assigning hybridization to an atom

• If 4 groups → sp³

• If 3 groups → sp²

• If 2 groups → sp

Electron pairs are counted as a group, an odd electron is not counted as a group

What does the sign of ΔG tell us about K

What does a ΔG value of -4.09 mean

means that the reaction is spontaneous and exergonic under standard conditions. This indicates that the reaction releases free energy and will naturally proceed forward to form products without needing external energy.

What is the relationship between ΔS, ΔH, and ΔG

What sign of ΔS means more disorder

A positive sign

In an acid-base reaction which side does the equilibrium side go towards

Goes towards the side of the weaker acid or weaker base

What shows that an acid is stronger than the other?

• Increased size of A → going down the periodic table

• Stronger electronegativity

• More resonance

• Smaller pKa or greater Ka

• Cations - ones with positive charges are stronger

What is the names of Alkanes from C1 to C10

What happens to boiling point and melting point as you increase the number of carbons

The boiling point increases and the melting point increases

What characteristics do branched alkane isomers have

• They have smaller surface areas → smaller London attractions and lower boiling points

• They are more compact → better crystal packing and higher melting points

True or False: The labels primary, secondary, and tertiary only apply to sp³ carbons and hydrogens on those carbons. Not other hybridizations

True

What is an alkyl group

An alkyl group is an alkane fragment that is missing one hydrogen atom and is represented by the generic symbol R

What are the different Alkyl groups

In newman projects where do the wedge marks go and the dashed ones

The wedge ones go to the rights and dashed ones go to the left

What is more stable in newman projects a staggered or eclipse

staggered

For each of the following energy values give a description of a phenomenon the value relates to

• 2.9 kcal/mol

• 10.8 kcal/mol

• -4.1 kcal/mol

What happens to ΔH as you go down this series:

Decreases

Out of primary carbon, secondary carbon, and tertiary carbon which one is the highest/lowest in energy and the most stable/least stable

• Most stable and lowest in energy: tertiary carbon

• Least stable and greatest in energy: primary carbon

What is the range values for C-H bonds. What happens when you increase the amount of carbons

96 to 105 kcal/mol. The bond energy decreases

What is the range of values for C-C bonds. What happens as you increase the amount of carbons

78 to 90 kcal/mol. The bond energy decreases.

What is hyperconjugation

p-Orbital (with a single e) overlaps with bonding molecular orbital of the neighboring C-H (or any other) bond

How does hyperconjugation look like for an ethyl radical, Isopropyl, and ter-butyl

What asked to draw a transition state what do you have to include

No formal charges, dipole charges, and brackets with this sign on upper right

When doing a halogenation what happens to ΔH values from the initiation, propagation, and termination phase

Initiation: ΔH values are at their lowest

Propagation: ΔH values increase at their highest

Termination: ΔH values are not high but not low

Which out of the halogens has the higest reactivity and which one has the lowest

F has the highest and I has the lowest because its endothermic

What does a halogenation need in order to occur. If a halogenation has Iodine will the reaction occur

It needs heat or light, if there is none there won’t be a reaction. If there is iodine no reaction will occur because its endothermic

What level of carbon does Br atoms connect to because they are very seletive

They attach to tertiary carbons

Where do fluorine atoms connect to because they are unselective

They connect to the one will the lowest amount of hydogren to make it lose one. Then it connects at the end and make it lose a hydrogen

When looking for the level of a carbon atom what do you look for.

Look for the amount of carbons it is connected to

• 4 carbons - quaternary

• 3 carbons - tertiary → most stable

• 2 carbons - secondary

• 1 carbon - primary

When labeling the primary, secondary, and tertiary hydrogens, what do you look for

Count the number of hydrogens connected to the carbon

• 3 hydrogens - primary

• 2 hydrogens - secondary

• 1 hydrogen - tertiary

How do you identify how many constitutional isomers can form

Find a line of symmetry and count how many carbons there are. If there is no symmetry then the amount of carbons is the amount of constitutional isomers.

Where do chlorine connect when in a halogenation

connects at the end of the structure or beginning

When doing curve arrow notation where do electrons flow

They flow from high electron density to low electron density (nuc→elec)

What shape geometrys have a sp³, sp², sp

What do chair confirmations for A and B look like. What is axial and what is equatorial

From a ring diagram to a chair confirmation what happens to wedge and dash

Wedges → Up direction

Dash → Down direction

When going from A to B, what happens to the elements

Going from A to B they move clockwise. They go counter-clock wise from B to A. Axial also turn to equatorial. Equatorial turn to axial

When doing chair confirmations what is a good pattern to remember which does axial and equatorial for cis and trans

• Cis: Both point in the same direction

• Trans: Both point in opposite direction

When making a more stable chair confirmation, how do you know what to put equatorial and axial.

The larger atom should be equatorial. Larger going down periodic table. CH3 is also larger than trigonal planar oxygens

Which ring is more stable and why

The more CH₂ the more stable the ring is as CH₂ relieves ring strain, making it more stable (adds 157.4 kcal/mol to heat of combustion -→ lower heat of combustion → more stable)

Between two chairs, where does equilibrium lie

Towards more stable ring

How do you use this table

When drawing chair conformations, the amount of delta G is based on which groups are axial. The one with lower delta G is more stable

What do you look for in ring strain

• Bond angle especially in small rings

• Eclipsing

• Transanular, especially in medium sized rings

How to find the configuration at a sterocenter

1.) Identify priority of atoms (higher atomic # wins —> CH₃ below CH₂CH₃)

2.) See which way you read

• Clockwise: R

• Counter-clock wise: S

3.) If the lowest priority group is not in the back then switch configuration

How to know two atoms are enantiomers

Same connectivity and mirror image of each other. If stereocenter the configuration should be the opposite (RR → SS) or (SS → RR)

Are the C-C bonds in a chair staggered or eclipsed?

Staggered. A boat has eclipsing interactions

Does a ring flip create a difference configurational steroisomer

No it produces another conformation of the same molecule

How do you identify R/S on a chair

Identify if attachments are wedges or dashes → redraw as ring → assign priorities → determine R/S

What are constitutional isomers

Same molecular formula but different atom connectivity

What are diastereomers

Same connectivity but different configurations without being mirror images. Some wedges turn to dashes, but not all. (RS → SS) or (RS → RR).

Example: cis/trans (or E/Z) isomers across a double bond or in a ring structure are classic examples of diastereomers that do not rely on tetrahedral chiral/stereocenters.

How do recognize identical molecules

Nothing changed, just the way you view it

(RR → RR) or (SS → SS)

If configurations are RS → SR. How do you know they are enantiomers or meso

If there is symmetry then its meso, if no symmetry then enantiomers

What is a meso compound

An achiral molecule containing sterocenters whose overall symmetry make its mirror image superimposable

How do you solve for entantiometric excess

% of major enantiomer - % minor enantiomer

How to find how many stereoisomers there are

2^{number of stereocenters} make sure there are no duplicates, if there is symmetry they are identical

What do the horizontal bonds of a Fishcer projection represent

Bonds pointing toward the viewer

What do the vertical bonds of a Fischer projection represent

Bonds pointing away from the viewer

What happens if you exchange two groups at a stereocenter, in fisher

One exchange inverts configuration, two exchanges preserve it

What is levoratory and dextrorotary

• Levoratory: rotates light towards the left (-)

• Dextrorotary: rotates light towards right (+)

If you have two pure enantiomers, how do their optical rotations occur

Equal magnitude and opposite sign

What is the optical rotation of a racemic mixture or pure meso compound

Zero

• Racemic: cancellation between enantiomers

• Meso: achiral

How do you find major % and minor % from enantiomeric excess

How do enantiomers compare in physical properties (density, melting point, boiling point, solubility)

Same ordinary physical properties. But they have opposite optical rotations so they could have different interactions with chiral substances

Does S_n2 inversion always change R into S

Not always. Geometry inverts but the priority of the groups attached may change, it depends

What is a nucleophile

An electron pair donor that attacks an electrophilic atom such as carbon

What is the difference between a base and nucleophile

A base attacks a proton. In substitution, a nucleophile attacks an eletrophilic carbon

Why is carbon attached to a halogen electrophilic

The halogen gives the carbon a partial positive charge

How many elementary steps does S_n2 have?

One transition step, bond formation and bond breaking occurs together

What is the rate law for Sn2

What is the Sn2 substrate reactivity trend? (Substrate - carbon connected to leaving group)

Why do tertiary substrates not undergo ordinary Sn2

Steric crowding prevents backside attack

What sterochemical change occurs during Sn2

Backside attack causes inversion of geometry at the reaction carbon

Does Sn2 have a carbocation intermediate

No one transition state and no intermediate

How do Sn2 transition state energies compare for R and S reactants

They are equal under otherwise identical achiral reaction conditions

What are the trends for comparing the strengths of leaving groups and nucleophiles

What is the connection between basicity and leaving group ability

Weaker bases are generally better leaving groups

What is the trend for bases on periodic table

Why does halide nucleophilicity reverse between solvent types

Protic solvents hydrogen bond strongly to smaller anions, decreasing their availability. Aprotic solvents do not provide the same hydrogen bond solvation

What are protic solvents

Polar O-H or N-H or H-F bond like Water, Methanol(CH3OH), or ethanol (CH3CH2OH), IPA

What are the aprotic solvents

Acetone, acetonitrile, DMF, DMSO, HMPA, Nitromethane