Fulton content sem1 Flashcards
(61 cards)
Define heats of hydrogenation
Heat evolved when a mole of the compound is completely hydrogenated
What is the heat of hydrogenation usually for a double bond
Usually about 120kJ mol-1 for every double bond
Why is benzene so stable
All 6 carbon are sp2 hybridised - each carbon has a p-orbital perpendicular to the plane of the ring. Each of the p orbitals contains a single electron. Partial overlap of the p orbitals allows the electrons to delocalise around the entire ring
What must a molecule have to be aromatic
Must be cyclic
Must be planar
Must be cyclically conjugated
Must obey huckels rule
Must possess an induced ring current when placed in a magnetic field
What does cyclically conjugated mean
each atom of the ring(s) must have p-orbital perpendicular to the plane of the ring to allow complete delocalization of the
π-electrons
What is huckels rule
Must possess (4n+2) pi electrons (n=0,1,2 etc)
2,6,10,14 electrons
Describe induced ring currents in benzene
Induced ring currents in benzene happen when its electrons move in response to an external magnetic field. This creates small currents in the ring structure, causing benzene to be repelled by h (a property called diamagnetism).
What effect does magnetic field have on the protons
The greater the magnetic field experienced, the further downfield those protons appear in the 1H NMR spectrum
What is the best indication of whether a compound is aromatic
Detection of a ring current
All aromatic compounds possess a ring current
Describe the two step mechanism for electrophilic aromatic substitution of benzene
1) The p-electrons of the ring attack the electrophile to yield a resonance stabilized carbocation - wheland intermediate
2) Substitution is completed by loss of H+
What is the product and reagents when nitroaromatics are reduced
Nitroaromatics can be reduced to aromatic amines
Reagent - 1) SnCl2/HCl 2) OH-
What is the electrophilic in aromatic sulfonation
Electrophilic is SO3 or HSO3+
What reagents can be used to produce the reverse reaction of aromatic sulfonation
To make the reaction go in opposite direction use steam or dilute H2SO4
When does halogenating of aromatic rings occur and describe the reagents for chlorine, bromine and iodine
Halogenating occurs with either Cl2, Br2 or I2 in the presence of a Lewis acid catalyst
For bromination use Br2 and FeBr3
For chlorinations use Cl2 and FeCl3
For iodinations use I2 and CuCl2
What is Friedel crafts alkylation
Friedel crafts alkylation a H is substituted for an alkyl R group
Using R-X and AlCl3
Where R-X must be and alkyl halide
Why is Friedel crafts alkylations not usually useful in synthesis
i) the alkyl group “activates” the ring towards further attack and the 2nd group substitutes on the ring more easily than the first
ii) When primary alkyl halides are used, the electrophile may rearrange to give unexpected products
Why are Friedel crafts acylations synthetically much more useful
i) The acyl group has a deactivating
influence on the ring and further substitutions are inhibited
ii) Ketones can be easily reduced
What reagent is needed to go from a carboxylic acid to a acid chloride
SOCl2
If a substituent is already present on the benzene ring, what can it influence
The rate at which further substitution reactions occur
The location at which the next substituent is introduced
What are activating substituents and give examples
Groups like -NH2 and -OH
They make the benzene ring more reactive towards electrophilic by donating a pair of electrons, the ring contains more electron density therefore increase the rate of reaction
What are and give examples of deactivating substituents
Groups like -NO2 are deactivating substituents. It makes the benzene ring less reactive towards electrophilic by withdrawing electrons from the ring. The ring contains less electron density therefore decreased rate of reaction
Describe inductive effects
These involve electron donation or electron withdrawal through σ bonds
Do electronegative elements have a positive or negative inductive effect
Negative inductive effects
Do electropositive elements have a positive or negative inductive effect
Positive inductive effects
