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CHEM 110 > Block 4 > Flashcards

Block 4 Flashcards

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1
Q

Describe the functional group alkenes: what types of bonds, what is hybridisation of the carbons, bond angle and orientation?

A

two carbon-carbon bonds, one is sigma and one is pi, both carbons are sp2 hybridised so double bond is planar with bond angles of ~120 degrees

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2
Q

The alkene double bond is considered to be: … with 2 electrons in the … bond and 2 electrons in the … bond

A

electron rich, sigma and pi

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3
Q

State the two reactions that prepare alkenes

A

elimination reactions - acid catalysed (H2SO4) dehydration of alcohols and base promoted (KOH/NaOH in ethanol) removal of HX from alkyl halides.

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4
Q

What is Saytzeff rule

A

The major product is the most substituted alkene; that is the alkene with the least number of hydrogens directly attached to the carbons of the C=C

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5
Q

Why is the major product of elimination favoured?

A

Less hydrogens attached to the Cs in the double bond means that is it is a lower energy/ stabler product

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6
Q

Why do alkenes generally undergo addition reactions

A

the double bond acts as a nucleophile as it is e- rich

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7
Q

Describe hydrogenation of alkene (sometimes called reduction)

A

alkene + H2 - Pt catalyst -> alkane

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8
Q

What is the stereochemistry of hydrogenation of alkene

A

Syn - Hs are added to the same side of the alkane forming cis- alkanes

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9
Q

Halogenation has the reagents of Br2, Cl2. What is the mechanism of addition

A
  1. Halogen acts as an electrophile with the electrons in the alkene double bond polarising the Br-Br bond. Double bond attacks the Br forming a carbocation +Br
  2. The remaining halide ion acts as a nucleophile and adds to the carbocation intermediate
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10
Q

What is the stereochemistry of halogenation of an alkene and why is it that way

A

Anti-stereochemistry = only gives the trans product. This is because the carbocation intermediate is actually a bromonium ion intermediate that has Br bonded to the 2 carbons so that the second Br can only attack from the top

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11
Q

If there are other nucleophiles present at part 2 of the halogenation mechanism these can

A

compete with the other halide ion and give different products.

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12
Q

Markovnikovs rule: Addition of an unsymmetrical reagent (H-Z) to an unsymmetrical alkene gives what as the major product

A

the compound where the electropositive part of the reagent (usually H+) has bonded to the carbon onf the C=C that is directly bonded to the greater number of hydrogen atoms.

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13
Q

describe the addition of HZ to alkene mechanism

A
  1. double bond attacks H+ forming carbocation.

2. addition of the nucleophile to the C+ and then charges are balanced by loss of proton if required

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14
Q

Why is the major product of addition favoured?

A

the major product is formed from the more stable carbocation which is in order from 3>2>1. More stable= more readily formed so more of the product formed.

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15
Q

Describe the function group of alkynes : what types of bonds, what is hybridisation of the carbons, bond angle and orientation?

A

2 sp carbons with one sigma bond and 2 pi bonds with bond angles of 180 °

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16
Q

Describe how alkynes are prepared : reaction, reagents, mechanism

A

dehalogenation of a dihaloalkane using NaOH/ KOH in ethanol. 2 moles of HX are removed to form an alkyne.

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17
Q

Benzene (an aromatic compound) does not undergo reactions typical of alkenes (addition) because of

A

The conjugated double bonds in the structure of benzene giving it extra stability = resonance energy. Electrons are shared around the ring.

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18
Q

Cyclic hydrocarbons can be aromatic if they contain

A

4N+ 2 e where N is a while number

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19
Q

How do you say monosubstituted benzenes

A

substituent name + benzene = bromobenzene

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20
Q

what is phenol

A

benzene + alcohol

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21
Q

What is ortho

A

1,2

22
Q

What is meta

A

1,3

23
Q

What is para

A

1,4

24
Q

How do you make an alkane from an alkyne

A

add pt or palladium + H2

25
Q

How do you make a Z alkene (Hs on the same side) from alkyne

A

add lindlar catalyst and H2

26
Q

How do you make an E alkene (Hs added trans) from alkyne

A

add Li/liq ammonia and then water

27
Q

How do you make a ketone from alkyne

A

using aqueous H2SO4 or HgSO4. IT adds one mole of water

28
Q

What is the reaction mechanism of addition of water (hydration) to alkyne

A

Markovnikov addition of OH and H which is unstable and forms a tautometric equilibrium between that (the enol) and the ketocompound which is the final product.

29
Q

How to form an alkynide anion and why does it happen

A

A strong base can remove the hydrogen on the terminal carbon of an alkyne as the terminal sp carbon is weakly acidic.

30
Q

Forming an alkynide anion is useful for

A

carbon chain extension

31
Q

What is an electrophile

A

E+, something that loves electrons. e- attack it

32
Q

Describe the mechanism of electrophilic aromatic substitution (SeAr)

A
  1. double bond attacks the electrophile.
  2. wayland intermediates—> resonance stabilised cations form
  3. Proton loss
33
Q

What is the fast and slow step of SeAr

A

fast step is proton loss. slow step is electrophile attack.

34
Q

Describe whats happening to the charge in the resonance stabilised cation. What is another name for this

A

Moving an electron to the positive charge that forms when two things are bonded to a carbon in the ring. The positive charge is only ortho or para to the Electrophile, never meta. Wayland intermediates equates to Resonance hybrid. Has a dotted half circle moon with plus charge in the middle.

35
Q

What is the stereochemistry of electrophilic (HZ or X2) addition reaction of an alkyne

A

Markovnikoffs rule is followed for HX addition, the reaction can be stopped after one addition and anti stereochemistry of addition is observed

36
Q

Why does a G substituent on the benzene ring direct ortho/para. What are the substituents that are op directing

A

It has lone pairs of electrons that can be donated.

OH, NH2, OCH3, X or R

37
Q

Why does a G substituent on the benzene ring direct meta. What are the substituents that are meta directing

A

It is electron withdrawing and has a positive end of a polar bond or a formal negative charge.
CN, COOH, COH, NO2

38
Q

What are the activating G substituents and why are they activating

A

OH, NH3,-OCH3 and R. They have electrons to donate by resonance. R donates them by induction

39
Q

What are the deactivating substituents and why are they deactivating

A

X, CN, COH, COOH and NO2 because they are electron withdrawing - the electropositive part of the polar bond is attached to the benzene.

40
Q

What is the explanation of substituent directing power for ortho para directors.

A

When an orthopara director is attached it has 4 resonance contributors for ortho/para but only 3 for meta. so more resonance contributors = more stable.

41
Q

What is the explanation of substituent directing power for meta directors.

A

For meta directors they have their positive end bonded to the benzene making only 2 resonance contributors possible (positive charge cannot be on the carbon bonded to the substituent for ortho/para but 3 for meta substituion.

42
Q

Tell 2 reactions that make haloalkanes

A

HX addition to an alkene (adding anti or alcohol with SOCl2 which swaps the OH for a Cl.

43
Q

What is the order of increasing ease of substitution for halogens

A

The bigger halogen it is the easier it is to leave -> starting from Cl, Br, I = most easy

44
Q

What is the rate order for Sn1 and E1 reactions of a haloalkane and what other thing do they have in common

A

rate is first order because the mechanism is 2 steps by a carbocation intermediate with the first step only having the RX as the reactant.

45
Q

What is the stereochemistry of a Sn1 mechanism

A

Sn1 mechanism have racemic mixtures of S and R enantiomers when the haloalkane is chiral.

46
Q

What is the stereochemistry of a Sn2 mechanism

A

For Sn2 using chiral but non racemic haloalkane results in the products becoming the opposite R/S due to inversion of configuration at the site of substitution

47
Q

The reaction order usually favours the more stable carbocation so tertiary haloalkanes for elimination and substitution of haloalkane. Which mechanism is different

A

Sn2 because transition state is less crowded for 1 and 2 so Ea is lower.

48
Q

What is the rate law for bimolecular mechanisms E2 and Sn2

A

rate = [RX][Nu-]

49
Q

Describe the mechanism of E2 elimination of a haloalkane

A

The base attacks the H, removing the proton, the C=C is formed and the C-X bond is broken all in one step.

50
Q

What are the rules for filling out the table telling you about the competing reactions

A
  1. Sn2 fills the top two rows with single Sn2 being the T
  2. E2 is only in basic column
  3. E1 is only in the bottom row
  4. Sn1 pairs with E1 and Sn2 of 2’
51
Q

Grignard reagents are made using

A

aryl /akyl halide + Mg in dry diethylether to make a carbon + Mg + halide which makes the carbon a carbanion

CHEM 110 (8 decks)

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