Chapter 10: Organics (part 1 - fundamentals of organic chemistry)) Flashcards
What is the property catenation?
the ability of carbon atoms to join together and form chains and rngs
What is alkanes?
- a family of hydrocarbons
- all straight-chains
What is the general formula of alkanes?
What is a hydrocarbon?
a compound containing only carbon and hydrogen
What is empirical formula?
the simplest whole number ratio of the elements present in a compound
What is molecular formula?
the total number of atoms of each element present in a molecule of the compound. Is an integer multiple of the empirical formula
What is a full structural formula?
also called a displayed or graphic formula
shows all the atoms and bonds in a molecule
What is a condensed structural formula?
the simplest representation that shows how the atoms are joined together in a molecule
What is this?
The skeletal formula of butane
What is a homologous series?
a series of compounds that have the same functional group
each differs from the next by a common structural unit (usually -CH₂-)
What is a funcional group?
The atom or group of atoms in a molecule that gives it its characteristic chemical properties - this is the reactive part of a molecule
Why is there not really a functional group in alkanes?
beause only single C-C and C-H bonds are present
Functional group of Alkane
none
Functional group of alkene
Functional group of alkynes
Functional group of alcohol
Functional group of ether
Functional group of aldehyde
Functional group of ketone
Functional group of carboxylic acid
Functional group of halogenoalkane
Functional group of amine
Functional group of ester
Functional group of nitrile
Functional group of amide
General name of alkane
General name of alkene
General name of alkynes
General name of alcohol
General name of aldehyde
General name of ketone
General name of carboxylic acid
General name of halogenoalkane
General name of ester
What is the molecular formula of Benzene?
C₆H₆
What is the molecular formula of Benzene?
C₆H₆
What is the molecular formula of Benzene?
C₆H₆
What does benzene look like?
its molecules have a planar, hexagonal ring of 6 carbon atoms with one hydrogen atom joined to each
What are compounds containing and without bezene rings described as?
with: aromatic
without: aliphatic
What is the functional group in aromatic compounds?
a benzene ring because it performs characteristic chemical reactions
How to interpret this?
There is a carbon atom and, if there is nothing else attached, a hydrogen atom at each vertex
How does the boiling point change as the number of carbon atoms in a molecule in any homologous series increase?
boiling point increases
Why does the boiling point of straight-chain alkanes increases when a methylene (-CH₂-) group is added?
Because the strength of the london forces between molecules increases as the relative molecular mass (number of electrons in the molecule) of the alkanes increases
They follow the same trend but why is the boiling point of alcohol higher?
because of hydrogen bonding between alcohol molecules
When can comparisons based on the relative molecular mass (and hence strength of london forces) be made and sometimes not?
can be made within a particular homologous series but not beween homologous series
Compare the boiling point of alcohols and ethers
alochols have higher boiling points than ethers
Compare the solubility of alcohol and ethers
alcohols are more soluble in water because alcohols can form hydrogen bonds to each other and also to water molecules
ethers have no hydrogen atoms bonded to oxygen atoms and so cannot form hydrogen bonds
How does the hydrocarbon chain of alcohol affect the solubility?
solubility decreases as the length of the hydrocarbon chain increases
Why are alkenes and alkynes called unsaturated compounds?
because tey contain C=C or C=O bonds
What compounds are not described as saturated?
alkanes and other compounds not containing multiple bonds
Is benzene unsaturated or saturated?
is described as an unsaturated hydrocarbon as the structure could be drawn with three C=C
General formula of Alkanes
General formula of Alkenes
General formula of alcohol
What can alcohols, halogenoalkanes and amines be classified as?
Primary, secondary or teritary
How are alcohols described as primary, secondary or tertiary?
depending on the number of carbon atoms attched to the carbon with the -OH group
What alcohol is ethanol?
Primary
because it has one carbon atom attached to the C with the -OH group
What alcohol is Propan-2-ol?
Secondary
because it has two carbon atoms attached to the C with the -OH group
What alcohol is 2-methylpropan-2-ol?
Tertiary
because it has 3 C atoms attached to the C with the -OH group
What alcohol is methanol?
primary
even though there are no other carbon attached to the C with the -OH group
What symbol can be used to represent primary, secondary and tertiary?
1˚, 2˚, 3˚
Halogenoalkanes described as primary, secondary or teriary depending on what?
depending on the number of carbon atoms (or alkyl groups) attached to the carbon atom of the C-X group
How are amines classified as primary, seconary or tertiary?
Primary: nitrogen atom is attached to one carbon atom and two hydrogen atoms
Secondary: if nitrogen is attached to two carbon atoms and one hydrogen atom
Tertiary: if nitrogen is attached to 3 carbon atoms and has no hydrogen atoms
What functional group does primary, secondary and tertiary amines contain?
What are structural isomers?
are two or more compounds that have the same molecular formula but different structural formulas - the atoms are joined together in different ways
What are the structural isomers with the molecular formula C₄H₁₀?
What are the structural isomers of C₅H₁₂?
What is not an isomers?
if the atoms are joined together in the same way
i.e. bent
Isomers of C₆H₁₄
How do the branched-chain isomer’s boiling points compare to straight-chain isomers?
Branched-chain isomers have lower boiling points
because the branches prevent te main chains from getting as close together, and so the london forces between the branched molecules are weaker
How can isomers of C₆H₁₂ be obtained?
remember this is an alkene, functional group is a double bond
By simply moving the double bond to a different position
What may be different about isomers besides their structure?
isomers may have different functional groups
What are functional group isomers?
isomers with different functional groups
esters are isomeric with carboxylic acids. So ethanoic acid and methyl methanoate are isomers; propanoic acid, methyl ethanoate and ethyl methanoate are isomers
What is the Kekulé benzene consist of?
a structure consists of a planar, hexagonal ring of carbon atoms, with a hydrogen atom joined to each carbon atom
this structure has alternating single and double bonds between the carbon atoms
What is alternating in the Kekulé bezene?
has alternating single and double bonds between the carbon atoms
What is a systematic name for the kekulé benzene?
cyclohexa-1,3,5-triene or 1,3,5-cyclohexatriene
Why was the kekulé benzene structure modified?
it was accepted for many years but eventually the weight of evidence against it became too great and a modified structure was proposed
What is the structure of benzene better represented as now?
each carbon atom seems to form just 3 bonds - two to C atoms and one to an H atom
What are the remaining elctrons of the modified benzene model represented as?
a circle in the centre of the structure
What is represented by the circle in the middle?
4 remaining electrons that are not localised between individual carbon atoms in double bonds but instead are spread over the whole ring
the ring of electrons is formed when p orbital soverlap side-on to form a π delocalised system
How is Benzene linked to leukemia?
it is recognised as a human carcinogen which can cause leukemia
the main source of exposure to benzene in Europe is from car exhaust fumes, but benzene is used in some industrial applications and some people are exposed to it at work
limits are usually set for occupational expsure to benzene but different countries set different limits on human exposure
What are the chemical and physical evidence for the structure of benzene?
Chemical - evidence from thermochemical evidence (enthlapy) and from the reactions of benzene (substitution not addition)
Physical - evidence from C-C bond lengths and the number of isomers of C₆H₄Cl₂
What is the evidence from carbon-carbon bond lengths for the structure of benzene?
In benzene
* all C-C bond lengths are equal and also intermediate in length between a C-C single bond and a C=C double bond
In kekulé benzene
* expected there would be 3 short C=C bonds and three longer C-C bonds
The delocalised structure of benzene suggests:
* a C-C bond length between that of a C-C bond and a C=C double bond because there are, on average 3 electrons (two from the single bond and one from the delocalised electrons) between each pair of carbon atoms compared with 4 electrons in a C=C bond and two in a C-C bond from the kekulé benzene
How are the bond lengths in benzene?
All C-C bond lengths are equal and also intermediate in legnth between a C-C single bond and a C=C double bond
What would be expected of the bonds if the structure of benzene were cyclohexa-1,3,5-triene (kekulé benzene)?
then it would be expected to have 3 short C=C bonds and 3 longer C-C bonds
What does the delocalised structure of benzene suggest?
- a C-C bond length between that of a C-C bond and a C=C double bond because there are, on average, three electrons (two from the single bond and one fromthe delocalised electrons) betetween each pair of carbon atoms compared with four electrons in a C=C bond and two in a C-C bond
What can X-ray crystallography determine?
C-C bond lengths, and others
When cyclohexene (C₆H₁₀) is heated with hydrogen in the presence of a nicket catalyst, what is formed?
cyclohexane C₆H₁₂
What reaction is it when cyclohexene (C₆H₁₀) is heated with hydrogen in the presence of a nicket catalyst to form cyclohexane C₆H₁₂?
an addition reaction in which hydrogen adds across the C=C bonds of the cyclohexene
and hydrogenation
What does the ring of delocalised electrons offer to benzene?
extra stability = lower enthalpy change to cyclhexane compared to if it was the kekulé structure with no delocalised electons
What thermochemical evidence is there for the structure of benzene?
- cyclohexene undergoes addition reaction and hydrogenation and turns into cyclohexane (C6H12) with the enthalpy change of -120kjmol-¹
- since cyclohexene only have on double bond, and kekulé benzene have 3, we would expect the enthalpy change for the complete hydrogenation of it to cyclohexane to be 3x-120 = -360kjmol-¹
- however the enthalpy change when benzene undergoes complete hydrogenation to cyclohexane is only -250kjmol-¹
- by comparing the enthalpy change, it is seen that the benzene is 155kjmol-¹ more stable than expected from the kekulé if it had the structure with alternating single and oduble carbon-carbon bonds
- the extra stability of benzene compared to the structure with alternating double and single bonds is due to the delocalisation of electrons
What evidence is there from the number of isomers of C₆H₄X₂ for the structure of benzene?
Further evidence for a delocalised structure from benzene comes from the examining the number of isomers that are possible for C₆H₄X₂ assuming a Kekulé structure and a delocalised structure
- only 3 isomers have ever been found for C₆H₄Cl₂ but the structure with alternating double and single bonds suggests that there should be 4
- the difference arises because two chlorine atoms that are on adjacent carbon atoms in the ring can either be separated by a C-C single bond or a C=C double bond, if benzene is assumed to have the kekulé structure
How does benzene not react like alkenes?
it does not undergo addition reactions under normal conditions and will not declorise bromine water
What reactions does benzene undergo?
substitution with involve replacement of one or more atoms on the ring with another atom or group
What evidence is there from the reactions of benzene for the structure of benzene?
- kekulé benzene would be expected to undergo addition reactions (like all other alkenes) and to decolorise bromine water
- however benzene (does not react like alkenes) does not undergo addition reactions under normal conditions and will not decolorise bromine water
- an addition reaction would involve the destruction of the delocalised system and the loss of stability associated with it (although the delocalised ring is only temperarily disrupted in reactions, it is restored so that the exta stability is not lost)
- therefore benzene undergoes substitution reactions which involve replacement of one or more atoms on the ring with another atom or group
