Organic chemistry Flashcards
whats the non cyclic and cyclic formula of alkanes
Non cyclic formula = CnH2n+2
Cyclic formula= CnH2n
describe alkane structure
- Most basic type of hydrocarbon- all bonds are C-C or C-H whith free rotation
- Bonds are spaced tetrahedrally around carbon atoms-
- all bond angles are 109.5
- All alkane names end in -ane
what ar the names of alkanes
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
Decane
what are alkyl groups
- Alkyl groups are alkanes missing a hydrogen atom
- E.g. when the molecule methane loses one H atom, it becomes CH3 known as a methyl group or CH2CH3 is known as an ethyl group
what are the physical properties of alkanes
- Non-polar molecules as almost all non-polar bonds
- Immiscible with water but soluble in most organic solvents (benzene, chloroform…)
- Volatility (how easy it evaporates) decreases with increasing chain length (i.e. the more carbons, the higher the boiling point)
- This is because…the more atoms there are, the greater the instantaneous dipole-induced dipole forces, the more energy required to separate molecules.
- Branching increases volatility, lowers the boiling point because smaller surface area available for interaction
Reactivity of alkanes
- Fairly unreactive
- non-polar and no sites to encourage attack
- Good fuels, combine with oxygen in an exothermic reaction
- Complete combustion- CH4 + 2O2 → CO2 + 2H20
- produces CO2
- Incomplete combustion- CH4 + 1½O2 → CO + 2H2O —- - Produces CO (carbon monoxide)
- The more C, the more energy produced
what are the equations of complete and incomplete combustion of alkanes
- Complete combustion- CH4 + 2O2 → CO2 + 2H20
- produces CO2
- Incomplete combustion- CH4 + 1½O2 → CO + 2H2O - Produces CO (carbon monoxide)
what are the steps to free radical substitution to form CH3Cl
-
Initiation
- The absorption of UV light causes the Cl-Cl bond to break (photodissociation)
- Homolytic fission:- Cl2 → Cl* + Cl*
- via UV absorption
- Homolytic fission of Cl2 and Br2 is exothermic
- I2 is endothermic
- F2 is explosive
- The X-X bond is weaker than the C-H or C-C bond and is therefore the one that undergoes homolytic cleavage
- Propagation
Cl* + CH4 → *CH3 + HCl
or
Cl2 + *CH3 → CH3Cl + *Cl
- Radicals used and regenerated - allows the reaction to continue
- Free radicals are highly reactive
- The number of free radicals in a propagation step is always maintained. If there is a free radical on the left of the equation there will be one on the right
- Termination
When a reaction occurs between free radicals, a product forms, but no new free radicals are formed. This type of reaction is called atermination stepbecause it tends to end the reaction. There are several termination steps in the chlorination of methane.
Cl* + *Cl → Cl2
or
Cl* + *CH3 → CH3Cl
or
CH3 + *CH3 → C2H6
- The reaction ceases when the supply of reagents is depleted
- Termination steps involve removal of the free radicals
- i.e. no radicals available to continue the chain
what type of fisiion is happening in free radical substitution
homolytic fission
what are alkenes and their general formula
- At least one C=C
- General formula CnH2n…(with one double bond)
describe saturated and unsaturated when talking about hydrocarbons
- Alkanes- Only single bonds, maximum number of hydrogen atoms - SATURATED
- Alkenes- At least one C=C double bond, fewer than maximum number of hydrogen atoms - UNSATURATED
how do you name alkenes
- Select the longest chain of C atoms that contains the double bond
- Number the chain starting from the end nearest the double bond
- Use a number to indicate the first carbon in the double bond
- Prefix with alkyl group
e.g 2-methylbut-2ene, oct-3-ene
Practice naming alkanes and alkenes and halogenoalkanes
Physical properties of alkenes (solubility, boiling point)
Boiling point
- Similar trends to alkanes
- Increases as molecular mass and chain length increase
- For structural isomers of alkenes, the greater the branching, the lower the boiling point
Solubility
- Non polar so insoluble in water but soluble in organic solvents
describe the reactivity of alkenes
- More reactive than relatively inert alkanes
- due to the presence of the double bond
what is the structure of the double bond like
- Remember C=1s2 2s2 2p2
Becomes: C= 1s2, 2sp2, 2p1 - Each carbon involved in the double bond uses three of its electrons in the formation of three σ-bonds, and one of its electrons in the formation of a π-bond
- C=C double bond- planar- 120 degree bond angle- restricted rotation around the double bond
what happens to the bonds when the alkene reacts
- The π-bond breaks
- The σ-bond remains intact
What mechanism doe alkenes use
electrophillic addition
Practice electrophillic addition
- The π bond electrons are attracted to the δ+ end of the electrophile causing the double bone to break
- A new bond forms between one of the carbon atoms and the δ+ atom
- The bond in the electrophile breaks by heterolytic fission
- This results in the formation of a carbocation and a nucleophile
- The nucleophile attacks the unstable carbocation and the carbocation accepts its electron pair as highly reactive
- Another new covalent bond is formed and an addition reaction has taken place overall
E.g Hydrogen bromide and ethene - C2H4(g) + HBr(g) → C2H5Br(l)
Whats the order of reactivity of the hydrogen halides
HI > HBr > HCl (most → least)
What are halogenoalkanes
Halogenoalkanes are similar to alkanes but with one or more of the hydrogen atoms replaced by a halogen.
How are halogenoalkanes classfied and which of these is most/least reactive
- Primary (1°): carbon attached to one other carbon
- Secondary (2°): carbon is attached to two other carbons
- Tertiary (3°): carbon is attached to three other carbons
The tertiary reacts fastest, followed by the secondary and the p.rimary reacts slowest.
what mechanisms do halogenoalkanes use
Nucleophillic substitution.
What are examples of nucleophiles
NH3 (ammonia), CN-(Cyanide), OH- (hydroxide)
what are the steps to nucleophilic substitustion
- Nucleophiles (Nu-) attack the carbon of a carbon–halogen (C–X) bond, because the electron pair on the nucleophile is attracted towards the small positive charge on the carbon.
- The electrons in the C–X bond are repelled as the Nu- approaches the carbon atom.
- The Nu- bonds to the carbon and the C–X bond breaks. The two electrons move to the halogen, forming a halide ion.
The halide is substituted, so this is a nucleophilic substitution reaction.
what are ketones and aldehydes
In an aldehyde, an H atom is attached to a carbonyl group- C=O group on end of chain
In a ketone, two carbon groups (R) are attached to a carbonyl group- C=O in middle of chain
- both have the same molecular formula
what are the suffix for aldehydes and ketones, give examples
Aldehydes- suffix (-al)
- ethanal, propanal, 3-methylbutanal
Ketones - suffix (one)
- Propanone, hexan-3-one
what are examples of carbonyl (CO) compounds in veterinary
- Carbohydrates,
- Retinal (3,7-dimethyl-9-(2,6,6-trimethyl cyclohex-1-en-yl)nona-2,4,6,8 tetraenal)
- Surplus of Acetyly coA in fat metabolism is converted to Acetotacetate, acetone, B-hydroxybutarate
- sex hormones, testosterone, progesterone
whats the solubility of aldehydes and ketones like, due to the carbonyl group
- Small aldehydes and ketones are soluble in water due to hydrogen bonding between a lone pair on the oxygen of the carbonyl group and the hydrogen of the water.
- As size increases, solubility decreases due to interference in hydrogen bonding by the hydrocarbon ‘tails’ of the aldehydes/ketones.
describe the polarity of Aldehydes and ketones
- The carbonyl group is polar due to the greater electronegativity of oxygen (3.4) than carbon (2.6). This influences the properties of aldehydes and ketones, such as solubility.
Describe the relationship between boiling points and intermoleular forces in alkanes/ aldehydes/ ketones/ alcohol
General increase in boiling points from alkanes → aldehydes/ketones and alcohols due to increased intermolecular forces between each type of molecule
- Alkanes are only held together only by IDID forces. These forces increase with the size/length of a molecule.
- The polar carbonyl group in aldehydes and ketones means that as well as IDID forces, these molecules are also held together by dipole–dipole interactions.
- Alcohols are held together by IDID forces and dipole–dipole interactions. In addition, these molecules can form hydrogen bonds with each other, due to the slightly positive hydrogen atom of the hydroxyl group
describe the reactivity of aldehydes and ketones
- Although double bonds require more energy to break than single bonds, compounds with double bonds tend to be more reactive as addition reactions are possible.
- The positive charge on the carbon atom makes it open to attack by nucleophiles.
- Aldehydes and ketones can be reduced, forming primary and secondary alcohols respectively.
- Aldehydes may also be oxidized to carboxylic acids.
How is an aldehyde reduced and give an example
- The aldehyde is warmed with the reducing agent
- Sodium borohydride, NaBH4(aq) or lithium aluminium hydride (LiAlH4) in ether are often used:
e.g. CH3CHO + 2[H] → CH3CH2OH
- This is an addition reaction as well as a reduction reaction as H2 is added to the C=O
- forms an alcohol
What is the mechanism used by aldehydes and ketones
Nucleophilic addition
describe the mechanism of nucleophilic addition
- Nucleophile, :H-, from NaBH4
- Attacks δ+ carbon of polar C=O bond,
- Lone pair on H- donates, and forms a new covalent bond.
- π-bond of C=O breaks,
- Forms a negatively charged intermediate…
- Lone pair on O-, donates to a H2O molecule,
- Forming a new covalent bond and hydroxide ion.
How can you distinguish between an aldehyde and ketone
- Oxidation reactions provide a way of differentiating between aldehydes and ketones:
- Aldehydes readily oxidised to carboxylic acids
- Ketones are only oxidised under very vigorous conditions
- Require a mild oxidising agent such as benects/ Fehlings soluution, Tollens reagent.
describe the tollens reagent
Aldehydes will reduce silver ions in Tollens reagent. Tollens reagent is made by adding a small amount of NaOH to aqueous silver nitrate.
- Ag+ + e- -> Ag (s)
describe the feelings/ Benedict’s solution
- Both contain copper(II) ions (Cu2+)
- Fehling’s/Benedict’s is added and the solution is warmed
- If positive, Cu2+ is reduced to brick-red Cu(I) oxide: Cu2O(s)
How do you test and identify carbonyls
- If identification is required, then the unknown compound must be reacted with Brady’s reagent (2,4-dinitrophenylhydrazine dissolved in acidified methanol).
- A bright orange or yellow precipitate will indicate the presence of aldehyde or ketone.
- If the precipitate is purified by recrystallisation, the melting point of the crystals can be measured and compared with tables of the melting points of 2,4-dinitrophenylhydra-zones of all the common aldehydes and ketones to identify the mystery compound
What are the different mechanism and where are they used
- Alkanes- free radical substitution
- alkenes - electrophilic adddition
- Halogenoalkanes- nucleophilic substitutiomn
- aldehydes and ketones- nucleophilic addition (redu)
- Benzene ring- Electrophillic substitution
- Acylchloride -Nucleophilic addition elimination
