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aliphatic hydrocarbons
open chains of carbon atoms, contains single or multiple C-C bonds
alicyclic hydrocarbons
closed ring of carbon atoms, contains single or multiple C-C bonds, H atoms not typically shown in structure
aromatic hydrocarbons
contains benzene ring
functional groups
responsible for chemical properties
homologous series
a family of compounds having the same functional group
can be represented by the same formula
similar chemical properties
similar preparation methods
degree of substitution
determined by number of alkyl groups bonded to an atom
primary (1°)
one alkyl group is attached to alpha carbon
alpha carbon
carbon next to functional group
secondary (2°)
two alkyl groups attached to alpha carbon
tertiary (3°)
three alkyl groups attached to alpha carbon
what is the melting point of alkanes?
low melting points
non-polar and contains intermolecular LDF
simple molecular structure
how does the melting point of alkane increase with number of carbon atoms?
increases. as the number of carbon atoms increases, the electron cloud size increases, the extent of polarisation of the electron cloud increases and the strength of the intermolecular LDF increases
how does the melting point of alkanes change with increasing degree of branching?
decreases. as the degree of branching increases, molecule becomes more spherical and the surface area of contact between molecules decreases. hence, the strength of intermolecular LDF decreases
are alkanes soluble in non-polar solvents?
Soluble. Weak LDF of attraction between non-polar molecules is similar in strength and compatible to the weak LDF between non-polar solvents
are alkabes soluble in polar solvents?
Insoluble. Strong hydrogen bonding between water molecules is incompatible to weak LDF of attraction between non-polar molecules
what is the melting point of alkenes?
low melting points
simple molecular structure
how does the melting point of alkenes change with increasing number of carbon atoms?
increases. as the number of carbon atoms increases, the electron cloud size increases and the extent of polarisation of the electron cloud increases. hence, the strength of intermolecular LDF increases.
how does the melting point change depending on cis and trans-isomers?
melting point of cis<trans. trans isomers can pack better in a crystal lattice and hence there is greater surface area of contact between trans-isomers than cis-isomers. there is greater extent of polarisation in trans-isomers than cis-isomers and the strength of intermolecular LDF is greater in trans than cis
how does the boiling point change depending on cis and trans isomers?
boiling point of trans<cis. As cis-isomers are polar while trans-isomers are non-polar, there are weaker intermolecular LDF for trans-isomers while cis-isomers have stronger intermolecular dipole-dipole attractions. Hence, the energy required to overcome intermolecular attractions is greater for dipole-dipole than LDF
However, trans>cis if carboxyl/hydroxyl groups are in close proximity in cis-isomers. With increased extent of intramolecular hydrogen bonding in cis than trans, there is a decreased extent of intermolecular hydrogen bonding in cis than trans, and the energy required to overcome the intermolecular hydrogen bonding is lesser for cis than trans.
are alkenes soluble in non-polar solvents?
Soluble. Weak LDF between non-polar molecules are similar in strength and compatible to the weak LDF of non-polar solvents
are alkenes soluble in polar solvents?
insoluble. Strong hydrogen bonding between water molecules is incompatible with the weak LDF between non-polar molecules
what is the melting point of arenes?
low melting point
non-polar with intermolecular LDF
simple molecular structure
are arenes soluble in non-polar solvents?
soluble. weak LDF between non-polar molecules is similar in strength and compatible with weak LDF between non-polar solvents.
are arenes soluble in polar solvents?
insoluble. strong hydrogen bonding between water molecules is incompatible with weak LDF between non-polar molecules
what is the melting point of halogenoalkanes?
higher than corresponding alkanes
polar and contains dipole-dipole and LDF
simple molecular structure
how does the melting point of halogenoalkanes change with the increase in size of halogen?
increases. as size of halogen increases, the electron cloud size increases and the extent of polarisation of the electron cloud size increases. hence, the strength of intermolecular LDF increases.
how does the melting point of halogenoalkanes change with increasing number of carbon atoms?
increases. as the number of carbon atoms increases, electron cloud size increases and the extent of polarisation of electron cloud increases. Hence, stronger intermolecular LDF.
are halogenoalkanes soluble in non-polar solvents?
soluble. weak LDF between non-polar hydrocarbon chain is similar in strength and compatible with weak LDF of non-polar solvents.
are halogenoalkanes soluble in polar solvents?
insoluble. strong hydrogen bonding between water molecules is incompatible with weak LDF between non-polar molecules
what is the melting point of hydroxyl (-OH) compounds?
MUCH higher than corresponding alkanes
polar and contains intermolecular LDF, dipole-dipole and hydrogen bonds
simple molecular structure
how does the melting point of hydroxyl compounds change with increasing degree of branching?
decreases. as degree of branching increases, molecule becomes more spherical and the surface area of the molecule decreases. Hence, the strength of the LDF of attraction decreases
how does the melting point of hydroxyl compounds change as the number of -OH groups increases?
increases. hydrogen bonding becomes more extensive and energy required to overcome the more extensive hydrogen bonding increases.
how does the melting point of hydroxyl compounds change as the number of carbon atoms increases?
increases. as number of carbon atoms increases, the electron cloud size increases and the extent of polarisation of electron cloud increases. Hence, there is stronger intermolecular LDF.
Are hydroxyl compounds soluble in non-polar solvents?
Not as soluble. Strong hydrogen bonding between alcohol molecules is not compatible to weak LDF between non-polar solvents.
Are hydroxyl compounds soluble in polar solvents?
soluble. Able to form intermolecular hydrogen bonding with water molecules
Solubility decreases as hydrocarbon chain increases as the hydrocarbon chain is non-polar. Hence, LDF between alcohol molecules becomes predominant and non-polar hydrocarbon chains interfere with hydrogen bonding between water molecules and themselves. Solvation of alcohol molecules with long hydrocarbon chains would involve hydrogen bond breaking between water molecules and is too endothermic a process for a positive entropy change of solution to counter.
hybridisation
hybridisation is a term used to describe the mixing of atomic orbitals to generate a set of new hybrid orbitals that are equivalent.
rules of hybridisation
only orbitals of similar energy level can be mixed to form hybrid orbitals
the number of hybrids formed equals the number of atomic orbitals mixed
all hybrid orbitals formed are similar in nature but with different orientation in space
what is the strength of covalent bond affected by?
s character. as s orbitals are closer to the nucleus, a more effective orbital overlap can be achieved, giving strong covalent bonds
isomerism
structures with the same molecular formula but different arrangement of atoms in their molecules
constitutional isomerism
same molecular formula but different structural formula
chain: different arrangement of C atoms (straight or branched) giving rise to different physical properties but same chemical properties
positional: different positions of the same functional group, giving rise to same chemical properties but different physical properties
functional group: different functional groups, giving rise to different physical and chemical properties
stereoisomerism
same molecular formula and same structural formula
cis-trans: restricted rotation due to C=C or ring structure with 2 different groups attached to each C of double bond or C atoms in a ring structure (max number = 2^n, n = number of C=C bonds)
Enantiomerism: non-superimposable mirror images with not plane of symmetry
E/Z isomerism
occurs when
restricted rotation due to double bond or ring structure
must have more than 2 different groups bonded to it
atom with higher atomic number has the higher priority
longer hydrocarbon chains have higher priority
enantiomerism
contains chiral carbon: sp3 hybridised carbon with 4 different groups
maximum number of enantiomers = 2^n, m = number of chiral carbons
no plane of symmetry
properties of enantiomers
identical physical properties except they rotate plane of plane-polarised light in equal but opposite directions
identical chemical properties except they interact differently with another chiral molecule
different biological properties, drug action
racemic mixture
equal proportions of both enantiomers where rotating power of one exactly cancels out of the other. does not rotate plane of plane-polarised light and are not optically active
diastereomers
have different configurations at one or more, but not all chiral carbons. not mirror images of each other.
meso compound
contains more than one chiral centre with a plane of symmetry. has mirror images that are super imposable but are not optically active
homolytic fission
even breaking of a covalent bond where one electron goes to each of the bonding atoms to form free radicals
heterolytic fission
uneven breaking of a covalent bond where both electrons go to one of the two bonding atoms to form positive and negative ions
free radical substitution
X2 (X = Cl or Br), UV light
observations: decolourisation of greenish-yellow Cl2/reddish-brown Br2, formation of white HCl/HBr that turn damp blue litmus paper red
alkanes are rather unreactive and unaffected by polar reagents due to its non-polar nature. saturated hydrocarbon does not contain any region of high electron density and therefore, do not attract electrophilic reagents
stability depends on number of alkyl groups attached to carbon atom with unpaired electron as it is electron-deficient. as alkyl groups are electron donating, they will enhance the stability of the alkyl radical by increasing the electron density around the carbon, dispersing charge and decreasing charge intensity
mono-substituted products
number of different hydrogen environment, where X2 needs to be the limiting reactant
electrophilic addition
Markonikov’s rule: when an asymmetrical alkene undergoes electrophilic addition, the more stable carbocation intermediate is formed. Hydrogen is added to carbon with more Hydrogen, forming more stable carbocation. electrophile then added to more stable carbocation, to carbon with more R groups, forming the major product (major product formed faster)
carbocation intermediate is trigonal planar in shape (step 1). in step 2, nucleophile can attack positive carbon atom of carbocation intermediate from top or bottom of plane with equal probability, forming a racemic mixture of enantiomers
electrophilic addition (alkene to dihalogenoalkane)
X2 in CCl4, r.t.p., dark (X = Cl or Br)
electrophilic addition (alkene to halogenoalcohol)
X2(aq), r.t.p., dark (X = Cl or Br)
electrophilic addition (alkene to halogenoalkane)
HX(g), r.t.p. (X = Cl or Br)
electrophilic addition (alkene to alcohol)
lab: cold concentrated H2SO4, followed by H2O, warm
industrial: H2O(g), H3PO4(l) on celite, 300 C, 70 atm (hydrolysis of esters too)
electrophilic substitution
concentrated HNO3(aq), concentrated H2SO4(aq), 55-60C
H2SO4 as catalyst and Bronsted-Lowry acid
aromatic character from the delocalisation of pi electrons is destroyed if electrophilic addition occurs. delocalisation of pi electrons bring extra stability, more than 2 unhybridised p- orbitals adjacent to each other. resonance structure forms intermediate between single and double bonds (non-integer bond order)
nucleophilic substitution
NaOH(aq)/KOH(aq), heat under reflux
Sn1: tertiary (unimolecular, overall first order, RX: first order, nucleophile: zero order) — rate of reaction increases as stability of carbocation increases where the more stable carbocation is formed due to lower activation energy — polar, protic solvents that stabilise carbocation intermediates by forming ion-dipole interactions
can form racemic mixture: nucleophile can form sp2 hybridised trigonal planar positive carbon atom of the carbocation intermediate from either side of the plane with equal probability. product formed is optically inactive
Sn2: primary as primary halogenoalkane have lower stearic hindrance (bimolecular, overall second order) — stearic factor of R groups: nucleophile must approach halogenoalkane molecule and attack the electron-deficient carbon from the side directly opposite the halogen atom. approach and attack is hindered by alkyl substituents bonded to the carbon atom — polar, aprotic solvents which have strong dipoles (without -OH or -NH) that can solvate the metal cation. the unsolvated nucleophile is free to attack electron-deficient carbon atom and reactivity of nucleophile to donate a pair of electrons to an electrophile is increased.
back-side attack: complete inversion of configuration with respect to chiral carbon atom. product formed is optically active
halogenoalkane to alkene
NaOH/KOH, ethanol as solvent, heat under reflux
alcohol to alkene
excess concentrated H2SO4, 170C OR H3PO4, 250C
alkene to alkane
H2(g), Ni catalyst, 150C OR H2(g), Pt or Pd catalyst, r.t.p.
alcohol to halogenoalkane
Dry HX, heat OR anhydrous PCl5, r.t.p.
primary alcohol to aldehyde
K2Cr2O7(aq), H2SO4(aq), heat with immediate distillation
primary alcohol to carboxylic acid
KMnO4(aq), H2SO4(aq), heat under reflux OR K2Cr2O7(aq), H2SO4(aq), heat with immediate distillation
carboxylic acid to primary alcohol
LiAlH4 in dry ether, r.t.p.
secondary alcohol to ketone
KMnO4(aq), H2SO4(aq), heat under reflux OR K2Cr2O7(aq), H2SO4(aq), heat with immediate distillation
ketone to secondary alcohol
LiAlH4 in dry ether, r.t.p. OR NaBH4 in methanol, r.t.p. OR H2(g), Ni catalyst, heat
Note
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