>Itis not easy forthe aromatics without alkyl groups to producealkenes by the crackingreactions.TheC-Cbond scission and dehydrogenation reactions ofthealkyl groups proceed for the aromatics with alkyl groups,whereas the polycyclic aromatics are formed by dehydrogenationand condensation reaction ofthe aromatic ring, thus the cokingtendency occurs.Thedifficulty-easinessotvariousseriessequencehydrocarbon cracking is as follows:Normal alkane>Isoalkane>Cycloalkanee(sixcarbocyclicringfivecarbocyclicring)>Aromatics
➢It is not easy for the aromatics without alkyl groups to produce alkenes by the cracking reactions. ➢The C-C bond scission and dehydrogenation reactions of the alkyl groups proceed for the aromatics with alkyl groups, whereas the polycyclic aromatics are formed by dehydrogenation and condensation reaction of the aromatic ring, thus the coking tendency occurs. The difficulty-easiness sequence of various series hydrocarbon cracking is as follows: Normal alkane > Isoalkane> Cycloalkane（six carbocyclic ring> five carbocyclic ring ) > Aromatics
二Reactionmechanism ofhydrocarbon cracking1.Free-radical reaction mechanismChaininitiation reactions are theprocess of free-radicalgenerationChain propagation reactions are the process of free-radicaltransformationChain termination reactions are the process of free-radicalextinctionto generate molecules
二、Reaction mechanism of hydrocarbon cracking 1. Free-radical reaction mechanism ➢ Chain initiation reactions are the process of free-radical generation ➢ Chain propagation reactions are the process of free-radical transformation ➢ Chain termination reactions are the process of free-radical extinction to generate molecules
Initiation reaction for three alkanes有关键的解离能/烷烃可能的链引发反应发生此反应的可能性(kJ /mol)CHsCHs-H-- CHs -+ H -小410CH3-CH3 - 2CH ·大368小C,H-H-→C,Hz + H 396~410CHs大CHCHs→ CH3 -+ CHs *354小381 ~396C,Hg-H-→ CHg -+ H .大350~357CH3CHz → CH3 ·+ C3H -CHio大345C,Hs--CHs→ 2CHs
Initiation reaction for three alkanes
R-R'→R·+ R'Chain initiationR. or R'.→ Alkenes + H.Chain propagationH+RH-H2+RHydrogen abstraction reactionsR'.+RH-RH+RR → R'. +AlkenesDecomposition reactionsR. → H.+ AlkenesH· +H→H2Chain terminationR·+R·→RREtc
Chain initiation Chain propagation R—R′ → R· + R′· R· or R′·→Alkenes + H· H· + RH → H2 + R· R′· + RH → R′H + R· R· → R′· + Alkenes R·→ H· + Alkenes Chain termination H· + H· → H2 R· + R· → R—R Etc. Hydrogen abstraction reactions Decomposition reactions
Initiation:H,C-CHsH,C+HCPropagation:HC+HC.CH,CH+HC-CHH.C-CH2H,C=CH2+HH+H,C-CH→H2+H,C-CH,H,C-CH,etc.Termination:H+HHH,C-CH2+H,C-→H,C=CHa+CHetc.Mechanism of ethane cracking reaction
Mechanism of ethane cracking reaction