Progetto: Rete e-Clil - liceobrocchi.vi.it · Alkanes •General characteristics •Nomenclature •Physical and chemical properties •Chemical reactions •Carbon hybridization

Progetto: Rete e-Clil

M.I.U.R. - Piano Nazionale Scuola Digitale

Capofila Rete Scuole: Liceo «G.B. Brocchi» di Bassano del Grappa (Vicenza)

Scuola della Rete:Istituto Statale di Istruzione Superiore «A. Parolini» di Bassano delGrappa

Lavoro di «e-clil Chemistry: ALKANES» prodotto dalla:Classe 2^AT indirizzo Tecnico Agrario – a.s. 2015/2016(Bernardi Marco, Bombieri Jacopo, Cunico Davide, Feltracco Nicole, Frighetto Davide,Girardi Emanuele, Girolimetto Mattia, Lago Massimo, Maresca Ivan, Parolin Sofia,

Pasqualotto Andrea, Pertile Michelle, Reginato Denise, Terzitta Alessio, Trento Matteo,

Vignoni Nicolò, Zen Andrea, Zonta Andrea, Zurlo Giacomo)

Docenti coordinatori: Cadò Francesca, Degetto Massimo, Iess Laura

Alkanes

• General characteristics

• Nomenclature

• Physical and chemical properties

• Chemical reactions

• Carbon hybridization

• Isomerism

Classe 2AT Indirizzo Tecnico Agrario – Istituto Statale di Istruzione Superiore «A. Parolini» di Bassano del Grappa – a.s. 2015/16

ALKANES

Alkanes are organic compounds.

They are composed of hydrogen and carbon – thatis why they are called hydrocarbons.

The atoms are joined up in a straight chain or ina ring (cycloalkanes).

CH4 (methane) is the simplest compound of thefamily of hydrocarbons.

Alkanes are saturated compounds: they arecharacterized by one carbon-to-carboncovalent single bond.


Alkanes are a homologous series; they have theCH2 repeating unit and their general brute formulais: CnH2n+2

ALKANES

METHANE CH4

ETHANE C2H6

PROPANE C3H8

BUTANE C4H10

PENTANE C5H12

HEXANE C6H14

HEPTANE C7H16

OCTANE C8H18


SOURCES AND INDUSTRIAL USES

The main sources of alkanes are:

• Natural gas

• Oil

Hydrocarbon compounds can contain from 1 to 30-40 carbon atoms.


NATURAL GASES

Natural gas contains only the mostvolatile alkanes, i.e. those with alower molecular weight.

It is mainly composed of methaneand a small quantity of ethane,propane and butane.

GPL

Propane and butane are used asfuels at a liquid state, and aremarketed in pressure cylinders withthe GPL acronym (liquefiedpetroleum gas).


OIL

Crude oil is a mixture of solid, liquid, gaseoushydrocarbons, which also contains small quantitiesof organic compounds.

In the mixture there are:

• alkanes

• cycloalkanes

• aromatic hydrocarbons

• rare alkenes

• no alkynes


NOMENCLATUREThe IUPAC nomenclature is the result of a series of rules:

Nr C Atoms Root name

1 Meth-

2 Eth-

3 Prop-

4 But-

5 Pent-

6 Hex-

7 Hept-

8 Oct-

10 carbon atoms: Dec-ane

Example:

1) find the longest chain of carbon atoms

2) build the name combining the root (= number of carbon atoms in the main chain; table on the left below) with suffix «-ane».


3) Use locator number to indicate the carbon atoms of the main chain:the numbering of carbon starts from the carbon closest to the firstbranch.

123

456

78910

4) It gives its name to the substituents or alkyl radicals(Hydrocarbon fragments obtained by removing an H group from thecompound: the name is obtained from the corresponding alkane byreplacing –ane with –yle.) Example:From alkane: CH4 is «methane» and the radical is –CH3 «methyle»From alkane: CH3-CH3 is «ethane» and the radical is –CH2-CH3 «ethyle»

5) Use the locator number to indicate the position of substituents(separated by a hyphen) along the main chain:

3-methyle; 4-methyle; 4-ethyle; 5-methyle; 6-methyle; 6-ethyle


6) Mention in alphabetical order the alkyls or substituents. If there aresubstituents that are repeated, group the position numbers andindicate with a prefix (di-, tri-, tetra-, penta-, hexa-...) the amount ofsubstituents of that type. If the substituents end in «-yle», eliminatethe final «e».

3-methyle

4-methyle

4-ethyle

5-methyle

6-methyle

6-ethyle

3,4,5,6-tetra-methyl

4,6-di-ethyl

4,6-diethyl-3,4,5,6-tetramethyl

Alphabetical order

There are 4 «methyl» substituents

There are 2 «ethyl» substituents

7) Assemble the full name before indicating the substituents followed bythe name of the main chain.

4,6-diethyl-3,4,5,6-tetramethyl-decane


PHYSICAL AND CHEMICAL PROPERTIES OF ALKANES

AND CYCLOALKANES


Physical status:

A lot of physical and chemical properties of alkanesdepend on the complexity of their molecules, namelyon the length of the main chain of carbon atoms andon its lateral branching.

• alkanes with 1,2,3 and 4 carbon atoms are aeriform

• alkanes with 5-17 carbon atoms are liquid• alkanes with 18 or more carbon atoms are solid

Density:

• increases with the length and the branching• in general, all alkanes have a lower density than

water and for this reason they float on water.


Melting and boiling pointsas for alkanes with 5 or more carbon atoms, eachextra carbon atom increases the temperature by20-30 °C.As their branching increases, the temperaturedecreases.

The properties can greatly vary according tothe type of structural isomerism (substanceswith the same brute formula, but with a differentarrangement of atoms).

SolubilityAlkanes are insoluble in water (hydrophobic) and inthe polar solvents, while they are soluble in non-polar solvents (benzene, ether, chloroform, carbontetrachloride).


ReactivityIn general, alkanes are compounds relativelyunreactive and rather stable (for this reason theywere called paraffines – from Latin par (um)barely + aff (īnis) connected + -in)

Their excellent stability is due to:

• the pure covalent bonds among carbon atoms,where the e- pair of the sigma bond is locatedand equally shared between two atoms;

• the C-H bonds have a low-polarity (the electro-negativity difference is low, around 0,24), whosepolarizing effects are reduced thanks to thesymmetric disposition of hydrogen atoms aroundthe carbon atoms, thus greatly increasing themolecule stability.


the simple C-C covalent bonds allow the carbonatoms to rotate around their bond in the open-chainmolecules. In this way, molecules, which are limitedby the repulsion of the same sign charges, may havedifferent structures (V.S.E.P.R. theory)

under standard conditions, alkanes do not react withbases, acids and oxidizing agents

alkanes can react with high temperature, ultravioletlight (UV) or in the presence of catalysts. The fewpossible reactions for alkanes are:

• Radical substitution (substitution of H for other elements, inparticular halogens → radical halogenation reaction)

• Combustion (oxidation reaction)

• Thermal and catalytic cracking (breaking of C-C and C-Hbonds)

• Halogenation


ALKANE REACTIONS

COMBUSTION (oxidation reaction)

HALOGENATION

RADICAL SUBTITUTION REACTIONS(thermal and/or catalytic cracking)

NUCLEOPHILIC SUBSTITUTION REACTIONS

ELETROPHILIC SUBSTITUTION REACTIONS


COMBUSTIONCombustion is the most common reaction. Hydrocarbonsburn in contact with air, producing carbon dioxide (CO2),water (H2O) and developing a huge quantity of heat,which is the main product of the oxidation reaction.

Representation of the methane reaction:

CH4+2O2 → CO2+2H2O+HEAT

In an airless environment, in addition to carbon dioxide (CO2),carbon monoxide (CO) may also be produced, which creates avery toxic environment.


HALOGENATIONAlkanes are the least reactive class of organic compoundsbecause they are made up of atoms characterized by verystrong, non-polar bonds.

CH4 + Cl2 → CH3Cl + HCl

CH3Cl + Cl2 → CH2Cl2 + HCl

CH2Cl2 + Cl2 → CHCl3 + HCl

CHCl3 + Cl2 → CCl4 + HCl

To obtain monohalogenatedcompounds it is necessaryto work in an excess ofalkane; to obtain a poli-halogenated compound it isnecessary to work in anexcess of halogen.

They can react with halogens F2 , Cl2 and Br2 (exothermic

reactions) thanks to:

Heat

Irradiation with visible or UV light

If there is an excess of halogens, the substitution reaction

continues, giving rise to a mixture of poly-halogenated

compounds


RADICAL SUBSTITUTIONThe radical substitution is divided into three stages:

Initiation: the reaction is started by the homolytic

cleavage (cracking) of X2 (x = any element) by heat or

by providing a quantum of energy (hn).

Propagation: the resulting radical reacts with the alkane

producing an alkyl radical, which, in turn, reacts with a

molecule of halogen, thus forming the alkyl halide and a

halogen radical.

Termination: when the resulting radicals recombine, the

reaction terminates.


Example:

Initiation (formation of a halogen radical by cracking):

Cl−Cl Cl● + Cl●T / hn

Propagation

Termination

Cl● + Cl● Cl−Cl

+ →

Alkyl halide

Alkyl radical

Halogenradicals


NUCLEOPHILIC AND ELECTROPHILIC

• Nucleophilic reagents are the electron-donors, i.e.:

negative ions

the groups of atoms having unshared electron pairs.

Nucleophilic reagents react with the site of a chemicalspecies in which there is an atom lacking electrons(electrophilic site).

• Electrophilic reagents are the electron-acceptors, i.e.:

positive ions (H+, H3O+, NO2

+; R+, carbocation)

atomic groups lacking electrons (AlCl3, BF3).

They react with atoms rich in electrons (nucleophilic site).


NUCLEOPHILIC SUBSTITUTION

The nucleophilic substitution is a substitution reaction in

which a nucleophile replaces, in a molecule, another

nucleophilic group, which takes the name of the leaving

group.

Leaving group= atom or group of atoms which breaks

away from a chemical substance leaving a residue or the

main part. This leaving group departs, with a pair of

electrons in heterolytic bond cleavage, as stable group.


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ELECTROPHILIC SUBSTITUTION

The electrophilic substitution is a chemical reactionin which an electrophile replaces another group (ora simple hydrogen atom) in a molecule

ELECTROPHILE SITE

NUCLEOPHILIC REAGENT

NUCLEOPHILIC LEAVING GROUP


CARBON HYBRIDIZATIONThe presence of one S orbital and three P orbitals in thesecond energetic level (valence level) allows the carbon to

combine the S orbital with theP orbital, thus getting sp1,sp2, sp³ hybrid orbitals,according to the number of Porbitals involved.

The sp³ hybrid orbital allowsthe molecule to direct thebonds in space arranging theatoms at the maximumdistance so as to minimizetheir repulsion (V.S.E.P.R.theory)


CHARACTERISTICS OF SIGMA BOND

The carbon atoms link up to form simple covalentbonds of σ (sigma) type: they are bonds that allow theatoms to rotate around the axis defined by the bondplane.

σ bondRotation

sp orbital


ISOMERISM

ISOMERISMSame brute formula but

different physical properties

can be:

STEREOISOMERISMdifferent atom

arrangement in space

STRUCTURAL OR CONSTITUTIONAL ISOMERS

Different sequences of atoms or groups of atoms in the molecules

subdivided into:

CHAIN ISOMERSsame number of carbon atoms

arranged in a different way

POSITION ISOMERSsame chain but different

position ofsubstituent atoms or

functional groups

FUNCTIONAL ISOMERSthe different atom

arrangement determines different functional

groups


STEREOISOMERISM

Stereoisomers are compounds whichpresent the same sequence ofmutually bonded atoms but differ inthe spatial orientation of theircomponent atoms.


Stereoisomerism is divided into:

Configurational

isomers:different arrangement of the

molecular components

Conformational

isomers:produce a rotation or a grouping of atoms around a C-C central simple

bond

Enantiomers(Optical isomers)

Diastereomers(Geometrical isomers)


Enantiomers and chiralityEnantiomers are molecular entities that are non-superimposable mirror images of each other.

Chirality is a property of enantiomerism.

To understand molecular chirality it is advisable touse molecular models.


It is found in organic molecules and is related to the tetrahedral form of the

C atom bonds.

The carbon atom becomes a chiral when it is bonded to four atoms or

groups of different atoms.

The chiral carbon atom is calledstereocenter.

Chirality


Diastereomers

A diastereomer is a stereoisomer which differs from an enantiomer.

Stereisomers that are not mirror images:

1-2, 3-4 pairs of enantiomers

1-4, 1-3, 2-3, 2-4 pairs of distereoisomers


Conformational isomerism

In Alkanes carbon atoms, having sp3 hybridization,can get a tetrahedral spatial configuration and themolecules are fairly free to move, namely to rotatearound C-C pure covalent bonds.

Therefore, elements can have different arrangementsin space, namely a different conformationalisomerism.

Isomers do not have thesame stability: the moleculeatoms tend to move inspace towards the 3D lessenergetic and more stableform.


Conformational Isomers

Owing to a free rotation around asimple C-C bond, they can havedifferent conformations. This rotationgives rise to an infinite number ofstructures called Conformers orRotamers.


The most important conformations are «the chair» and«the boat»: they are in equilibrium with each other so asto lean towards the conformation which is more stable.

Chair conformation

Boat conformation


In conformations, the hydrogen atoms can place themselves,in relation to the carbon atom ring, as follows:

• as axial substituents (hydrogen atoms perpendicular to theplane defined by carbon atoms – in red in the picture)

• as equatorials substituents (hydrogens on the same planedefined by carbon atoms – in blue in the picture).

Example of two chair conformations in the limit equilibriumbetween the space positions taken by the atoms.


GEOMETRIC ISOMERISM

dimethylcyclohexane dimethylcyclohexane

Geometric isomerism can be present inalkanes and cycloalkanes.The two pictures below show the differentposition of the two methyl groups:


Substituentson the same side of the plane

They are called ‘CIS’ if the two substituents (CH3) are on the same side of the ring plane.

dimethylcyclohexane


Substituentson opposite sides of the plane

They are called ‘TRANS’ if the two substituents are on opposite sides of the ring plane.

dimethylcyclohexane


STRUCTURAL ISOMERISM

It represents the differentsequence of atoms or groups of

atoms in the molecule.Structural isomerism can be:

CHAIN POSITION FUNCTIONAL

ISOMERISM ISOMERISM ISOMERISM


CHAIN ISOMERISM

Same number of carbon atoms (C4) but different chain structure

CH3-CH2-CH2-CH3

CH3-CH-CH3

CH3

CH2-CH2

CH2-CH2

Linear isomerism

Branchedisomerism

Cyclicisomerism


CHAIN ISOMERISM concerns the structure of the carbon skeleton,

namely the presence and positionof the branching or rings.

cyclobutene

Trimethyl-2-pentene ethylcyclohexane


POSITION ISOMERISMSame number of carbon atoms

(C4) but different position

POSITION ISOMERISM POSITION ISOMERISM

with ONE SUBSTITUENT with DOUBLE BONDS

CH2-CH2-CH3

|NH2

CH3-CH-CH3

|NH2

CH2=CH-CH2-CH3

CH3-CH=CH-CH3


POSITION ISOMERISM concerns the position ofmultiple bonds or groups containg atomsdifferent from carbon and hydrogen: ortho- (o),meta- (m), para- (p) position.

O-xilene m-xilene p-xilene


FUNCTION ISOMERISM

Isomers with the same brute formula, but withdifferent «functional groups».Example: C4H10O

CH3-CH2-CH2-CH2-OH alcohol

CH3-CH2-O-CH2-CH3 ether


Documents

Progetto: Rete e-Clil - liceobrocchi.vi.it · Alkanes •General characteristics •Nomenclature •Physical and chemical properties •Chemical reactions •Carbon hybridization