Alkanes Flashcards
What are alkanes?
Alkanes are saturated hydrocarbons that contain carbon atoms which are sp3 hybridised and are tetrahedrally bonded to either H atom or other C atoms
What is the general formula of alkanes?
CnH2n+2
What is the general formula of cycloalkanes?
CnH2n
What happens to the melting and boiling point of alkanes as the number of C atom increases and why?
Melting and boiling point increases as alkanes have simple covalent structures and as the number of C atoms increases, the number of electrons in the molecule increases and more energy is required to overcome the stronger id-id interactions between molecules
Are alkanes more or less dense than water and why?
Less dense, the packing of alkane molecules is less as compared to water molecules due to weaker id-id interactions between alkane molecules compared to the stronger hydrogen bonds between water molecules
What happens to the density of alkanes as the number of C atoms increases and why?
Density increases as when the number of carbon atom increases, molar mass increases, density increases as the increase in molar mass outweighs the increase in volume
Are alkanes soluble in water and why?
All alkanes are insoluble in water
- Alkanes cannot form effective interactions with water molecules
- During the dissolution of alkanes in water, the energy needed to overcome the stronger hydrogen bonds between water molecules is not compensated by the energy released when weaker id-id are formed between alkane and water molecules
Are alkanes generally reactive or unreactive and why?
Alkanes are generally unreactive towards polar and ionic reagents
- alkanes are non-polar thus unreactive towards polar reagents
- C-C and C-H bonds are very strong, a large amount of energy is required to overcome these strong bonds in order for reaction to occur
What are free radicals?
Species that posses at least one unpaired electron
What are the reagents and conditions for free radical substitution?
Condition: UV light
Reagents:
- for a large proportion of mono-substituted products, a large excess of hydrocarbon used
- for a large proportion of poly-substituted products, a large excess of halogen used
What is the reactivity of halogens in free radical substitutions (most reactive to least reactive)?
F2 > Cl2 > Br2 > I2
- F2 gives explosive reaction even in absence of UV light
- Cl or Br reaction proceeds rapidly only in UV light
- I2 no reaction under normal lab conditions
What is the stability of hydrocarbon radicals (most stable to least stable) and why?
tertiary radical > secondary radical > primary radical > methyl radical
- alkyl substituents which are electron-donating help stabilize the radical
What other factors can stabilize hydrocarbon radicals?
- Radicals can be stabilised by delocalised pi electron systems like benzene ring
-partially filled p orbital can participate in resonance with adjacent p orbitals, leading to greater stability
What factors destabilize hydrocarbon radicals?
- the greater the s character of the orbital, the half-filled orbital containing the free radical is held more closely to the nucleus, electron affinity of orbital increases, destabilizing the radical (alkyl > alkenyl / aryl > alkynyl)
- Higher electronegativity, greater electron affinity of the half-filled orbital, more unstable
- Smaller atom, electron-deficient orbital spread out over a smaller volume, decrease in stability
- destabilized by electron-withdrawing groups
What is cracking?
Cracking is a process where bigger hydrocarbon molecules are broken down into smaller molecules