Organic Chemisty (I) : Into to Org Chem - Halogen Derivatives Flashcards
What are
1. Homolysis
and
2. Heterolysis?
- Homolytic Fission.
Splitting of a single bond giving an equal share of bonding electrons to each resulting particle. (Results in formation of 2 radicals)
Single fish hook arrow - Heterolytic Fission.
Splitting of a bond giving an unequal share of bonding electrons to each resulting fragment. (Formation of Ions)
Full head on the arrow
What 3 species are Reaction Intermediates? Why?
- Free Radicals
- Carbocations
- Carbanions
All very reactive.
Odd electron in free radicals readily combines with an electron in another atom to form a stable bond.
Carbocations are electron-deficient and is hence a strong electrophile.
Carbanions are electron-rich and is hence a strong nucleophile.
Explain Inductive and Resonance Effects.
Inductive effect is the shifting of electrons in a bond in response to the electronegativity of nearby atoms.
Withdrawal or donation of electrons through a Sigma Bond.
Eg. Alkyl groups are electrons donating, Halogens are electron withdrawing
Resonance effect is the withdrawal or donation of electrons through a Pi Bond.
Eg. Electron withdrawing : -Y=Z, where Z atom is more electronegative than Y, like -C=O
Eg. Electron donating: -ӱ, where the y atom has a lone pair of electrons available for donation
What is Structural Isomerism? List all 3 examples
Same molecular formula but different structural formula.
- Positional Isomerism - Different positions of same functional group
- Chain Isomerism - Different Skeletal chain
- Functional Group Isomerism - Different functional groups
What is Stereoisomerism? List all 2 examples.
What is the formula to determine number of stereoisomers?
Same attachment of atoms to each other but different spatial arrangement.
- Geometric Isomerism. Caused by restriction of rotation due to double bonds or ring systems. Cis-Trans
- Optical isomerism. Present in molecules with no plane of symmetry. Molecules can exist as 2 non-superimposable mirror images (enantiomers). Chiral/Stereogenic centre - four different groups attached to carbon centre.
Maximum number of stereoisomers = 2^n, where n=total number of double bonds that fulfil geometric isomerism + no. of chiral centres
What are cases of optical inactivity?
- Presence of plane of symmetry in molecule despite Carbon being bonded to 4 different molecules (meso compounds). Achiral
- Racemic mixture. Equal proportion of +enantiomers (dextrorotatory rotate clockwise ) and -enantiomers (laevorotatory rotate anti-clockwise)
What are the 5 steps to naming compounds?
Step 1: Determine the principle functional group present
Step 2: Find and name the longest carbon chain with the principal functional group
Step 3: Number the chain starting from the end nearer to the principla functional group
Step 4: Name groups on the chain. Use (di-, tri-, tetra-, penta-,) if occur more than once. Designate location using numbers.
Step 5: Assemble with prefix, stem, suffix. Commas to separate numbers and hyphens to join letters and numbers. Substituent groups are in alphabetical order
Explain Alkane’s lack of chemical reactivity
- Carbon atoms’ saturation
2. C-C and C-H bonds are strong and fairly non-polar
Physical properties of Alkanes.
Include differences between straight chain and branced where necessary.
5 Types
- Physical state - 4 or less carbons are gases, 18 or more carbons are solids. Dispersion forces
- Boiling point - increase with increasing no. of carbon atoms. C increase, size of e- cloud increase, stronger dispersion forces, there more energy needed to overcome.
Branced alkanes have lower boiling points since they have smaller surface areas, hence dispersion forces are less efficient. - Density - increase with increasing carbon atoms. Because increasing strength of intermolecular forces = pack more closely together. Larger relative mass = density increase
- Melting Point - depend not only on size of the molecules but also how well the molecules fit into a crystal lattice
- Solubility - Low polarity and inability to form H-bonds so insoluble in water.
- Arrange the stability of alkyl radicals
and - Predict the proportion of products formed through free radical substitution of alkanes.
- Stability: Tertiary, Secondary , Primary and Methyl in decreasing order
- Proportion depends on number of hydrogen and stability of each type of radical formed. Eg. 6 primary hydrogens vs 2 secondary hydrogens in propane
and Eg. Secondary radicals more likely to form as compared to primary radicals
Reactants and products for combustion of alkanes
Reactants : Alkane and Oxygen
Products : Carbon Dioxide and Water
Explain the boiling point and melting points of alkenes.
B.P - increases as number of carbons increase due to increase in size of the electron cloud and hence its polarisability, leading to stronger dispersion forces.
Cis isomers tend to have higher boiling points, since they tend to be more polar.
M.P - Transisomers tend to have higher melting points since they pack more closely together due to higher symmetry of the trans isomer. Hence, dispersion forces work more effectively in holding the molecules together. More energy needed to overcome the forces of attraction between trans isomers = higher melting point
How to predict major and minor products in reactions?
Electron-donating groups that stabilise the carbocation by dispersing the positive charge are more likely to form (Eg. Methyl groups)
Conversely, electron-withdrawing groups that intensifies the positive charge on the central carbon atom destabilises the carbocation and is less likely to form. (Eg. Halogens)
What type of substitution does arenes undergo and why?
Electrophilic substitution. Rich source of pi electrons above and below the plane of the benzene ring which is highly reactive toward electrophiles.
Effects of substituents on the reactivity of the benzene ring. Which are activating and which are deactivating?
Electron-donating substituents increase the electron density of the benzene ring, thereby increasing the reactivity of the benzene ring. These are activating groups.
Vice versa for deactivating groups
Weakly Activating : 1. Alkyl Groups (Eg. CH3)
Strongly Activating : 2. -OH, -NH3, -OCH3
Deactivating : 1. Halogens (Eg. -Cl, -Br)
Strongly Deactivating: 2. -CHO, -NO2, -CN, -CO2H
Strength due to inductive and resonant effects
