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Core Chem 1A - Term 1 > Organic > Flashcards

Flashcards in Organic Deck (62):
1

Structure of acetone

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Structure of acetontrile

3

Structure of toluene

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Tetrahydrofuran structure

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Diethyl ether structure

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Pyridine structure

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Dimethylsulfoxide (DMSO) structure

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N,N-dimethylformamide (DMF) structure

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General structure of an acetal

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General structure of a carbonate

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General structure of a nitrile

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General structure of an ancyl chloride

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What is a conjugated system?

A system with two or more C=C bonds alternating with C-C bonds

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Briefly describe an SN1 reaction

- Substitution Nucleophillic Unimolecular

- Reaction passes through a carbocation intermediate

15

HA + H2O A- + H3O+

Ka =

[A-][H3O+] / [HA]

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pKa =

- log (Ka)

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pKa range of strong acids

-10 to 0

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pKa range of weak acids

0 to 16

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pKa range of very weak acids

16 to 50

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pH =

pKa + log ( [HA] / [A-] )

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What is a constitutional isomer?

Same molecular formula, different connectives

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What is a stereoisomer?

Same molecular formula, same connectives

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What are the two types of stereoisomers?

Configurational - 2 types

Conformational - Molecules can be converted into one another by rotation around a single bond

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What are the two types of configurational isomers?

Enantiomers - Non-superposable mirror images

Diastereoisomers - Non-superposable, not mirror images
eg. E/Z isomerism
'One end stays the same, the other is mirrored (wedge and dash switch)'

25

What is a chiral molecule?

A molecule that has a non-superposable mirror image. Has none of the following symmetry elements: A mirror plane, a centre of inversion, a rotation-reflection axis

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What is an achiral molecule?

A molecule that has a superposable mirror image

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What is a prochiral molecule?

A molecule that can be made chiral by swapping/adding one group

28

Measurement of optical activity formula

observed angle of rotation (o) / [path length (dm) x concentration (g ml-1)]

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How to decide if an isomer is E or Z

Find highest priority group on each carbon

- Higher atomic number

2 highest priority groups are on the same side is the Z isomer, when they are on the same side it is the E isomer

30

How can the configuration of 2 diastereoisomers (E/Z) be determined?

- They will differ in boiling point and the C13 and H1 NMR spectrums will differ, but it is hard to assign the configuration this way

- If crystals of either can be grown at low temperature, the crystal can be analysed by X-ray crystallography to determine the configuration

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How can the confoguration of 2 diastereoisomers (anti/syn) be determined?

- They are difficult to distinguish or separate

- Might be able to use low temperature microwave spectroscopy

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How to decide if an isomer is anti or syn

The syn isomer will have the 2 highest priority groups (or the 2 groups that have been added) on the same side, the anti isomer will have them on opposite sides

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What physical properties will 2 constitutional isomers differ in?

They will have different melting points, and their C13 and H1 NMR spectrums will differ (give examples)

34

On benzene, where do the ortho, meta and para groups lie?

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How can the configuration of 2 enantiomers be determined?

They can be distinguished by their ability in solution to rotate the plane of plane-polarised light

The actual configuration can be determined by X-ray crystalography of a single crystal sample of a pure enantiomers

36

More conjugation leads to...

Smaller HOMO-LUMO gap

ie absorbs lower energy

37

Factors determining the strength of the acid

- Most important: Stability of the conjugate base, a more stable conjugate base means a stronger acid

- Bond strength H-A, the easier the bond is broken, stronger the acid

- The solvent, the better the solvent is at stabilizing the ions formed, the stronger the acid

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What makes a more stable conjugate base?

- If the negative charge is on a more electronegative atom, the conjugate base will be more stable

- Delocalization of the negative charge, the more spread out the charge is, the more stable the conjugate base

39

Explain which is more acidic: phenol or nitrophenol

- The molecules both react with water, leaving a negative charge on the O

- Nitrophenol has an NO2 group which is electron-withdrawing, stabilizing the conjugate base with inductive effects

- So nitrophenol is more acidic

40

Describe the structure of nitrophenol


                           

HO - benzene - N+ = O
                            |
                           O-

41

Briefly describe an SN1 reaction

- Leaving group goes first

- Carbocation is formed

- Nucleophile attacks next

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Briefly describe an SN2 reaction

Nucleophile attacks at the same time as the leaving group goes

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The rate of an SN2 reaction depends on...

The concentration of the substrate and the concentration of the nucelophile

44

What increases the strength of inductive effects?

More electronegative atoms on the molecule, and these atoms being closer to the ionisation site

45

Small pKa =

a strong acid

46

Explain how to draw R and S isomers

Rank the groups from highest to lowest priority

Place them as follows:

R - starting from the top bond, go clockwise (rectus) putting the lowest priority group at the back

S - starting from the wedge, go anticlockwise (sinister) putting the lowest priority group at the back

47

What is topicity?

It relates atoms, groups or faces to each other within molecueles

48

What does constitutionally heterotopic mean?

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What does homotopic mean?

Atoms, groups or faces are related by an n-fold rotation axis, and the molecules are essentially the same

Eg. Gives the same molecule when Hs are replaced in turn, so the H atoms are homotopic

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What does enantiotopic mean?

Constitutionally identcal atoms, groups or faces in molecules which are related by a mirror plane, inversion centre or improper rotation axis. Replacement of one pair of enantiotopic groups forms one of a pair of enantiomers

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What does diastereotopic mean?

Constiutionally equivalent atoms, groups or faces of a molecule which are not symmetry related. Replacement of one or two diastereotopic atoms or groups results in the formation of one of a pair of diastereoisomers

52

The rate of an SN1 raction depends on...

- The carbon skeleton

- The leaving group

- Temperature

- Solvent

53

What substrates are likely to undergo SN1 mechanism?

Tertiary (excellent)
- Nu's attack is hindered and a carbocation is readily formed

Secondary (moderate)

Allylic (double bond) (good)

Benzylic (good)

Adj. lone pair

54

What substrates are likely to undergo SN2 mechanism?

Methyl (good)
- Nu can attack unhindered and a carbocation forms reluctantly

Primary (good)

Secondary (moderate)

Allylic (double bond) (good)

Benzylic

α-carbonyl

55

What makes a good leaving group?

A weak base / a strong acid

eg I, Cl

OH is never a leaving group (unless protonated with a string acid)

56

What makes a good nucleophile?

Charge
- Nucleophillicity increases with charge density

Electronegativity
- Nucleophillicity is the donation of an electron pair, the less electronegative the atom, the less 'tightly held' the electrons will be, so the better nucleophile

Solvent
- In polar protic solvents, nucleophillicity increases going down the periodic table. The capacity for hydrogen bonding is highest at the top of the periodic table, less hydrogen bonding means the nucleophile is less hindered

Steric hinderance
- Less sterically hindered (less bulky) nucleophiles are better

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Examples of polar protic solvents

Acetone, DMF, MeCN, DMSO

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Why is OH- a good nucleophile?

It is a strong base

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Characteristics of a hard nucleophile

- Small
- Charged
- Basic
- Low-energy HOMO
- Like to attack C=O
- Such as RO-, NH2-, MeLi

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Characteristics of a soft nucleophile

- Large
- Neutral
- Not basic
- High-energy HOMO
- Like to attack saturated carbon
- Such as RS-, I-, R3P

61

Why is tBu a bad electrophile?

Gives steric hinderance to the approach of the Nu, raising the energy of the transition state structure, giving a very slow rate of reaction

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